Continuity mistake: In the pre-credits sequence, when Danny and Sandy on walking on the beach, Danny is on the left and Sandy is on the right. However, in the next scene, when they kiss, they have suddenly changed positions. (00:00:40)
Continuity mistake: During the song Summer Lovin', Rizzo lies down on her side on a picnic bench and puts on sunglasses. In the next shot, when she kicks Sandy and another girl off the bench, the sunglasses are off. In the following shot, they are on again. (00:16:13)
Continuity mistake: Sandy walks over to meet the girls after doing a cartwheel. On Sandy's cheerleading uniform, there is a LONG red string right next to the letter on her shirt. Then, suddenly, the string disappears in the very next shot. Then in the scene after that it is back again, and then the string is back on her shirt. (00:22:20)
Continuity mistake: During the slumber party at Frenchie's, when Rizzo brings out the wine, check out the junk on the headboard. Later in the scene when Marty is putting on her robe, the headboard is clear and now has a black fan on the left. Then even later when Jan says "You ain't getting your hands on my ears" the headboard is cluttered again, with the black fan too. (00:27:35)
Continuity mistake: During the Greased Lightning song sequence, when Danny runs around the dream car, from one shot to the next, 2 white mattresses suddenly appear behind the car to aid the dancers jumping off. (00:39:55)
Continuity mistake: In the Greased Lightning dance scene, look just in front of the last white desk on the right side during the dream car segment. One of the dancers knocks a tyre off a stand and it falls in front of the desk. A shot or two later, the tyre is back up on the stand. (00:40:05)
Continuity mistake: When Kenickie says to Rizzo in the soda parlour, "How about if I finish with you, huh?" there is a tomato ketchup squeezer on his side of the table. In the next scene when Rizzo says, "Finish this!" and throws the glass of liquid at Kenickie, the ketchup squeezer has moved to her side of the table, and in the next scene, when Kenickie is covered in liquid, the ketchup squeezer is back in its normal position. (00:52:00)
Continuity mistake: At the beginning of the National Bandstand scene you see people filling the hat of the big "Rydell Ranger" figure with pieces of paper. During the next ten minutes the figure is often visible in the background and the hat is empty - nobody emptied it. After those ten minutes the hat has paper again and is then emptied. (00:59:00)
Continuity mistake: Once during the dancing scene you can see Vince Fontaine standing on Marty's left side, and then suddenly on her right. (01:10:16)
Continuity mistake: When Cha-Cha receives the award from Principal McGee at the end of "Born to Hand Jive" at the National Dance Off, Putzie is giving Jan a piggy-back next to the Principal. However, in the next scene, Putzie is giving Jan a piggy-back right next to Cha-Cha. (01:14:00)
Continuity mistake: After the Song "You're the one that I want" when Danny is doing the test your strength game you can see Frenchy running from the Ferris Wheel a second time when Rizzo and Kenickie start kissing again, when that scene happens just before the song. (01:44:50)
Continuity mistake: When the three T-Birds go to moon at the TV camera at the National Dance Off, Doody says "1, 2, 3, go!" and pulls down his trousers to his ankles. In the next scene, his trousers are suddenly pulled right up to his buttocks.
Continuity mistake: At Frenchie's sleepover when they are talking about piercing Sandy's ears, it shows Sandy holding the wine bottle and passing it to Jan. Then it cuts back to Sandy and she is still holding the wine, then it cuts back to Jan and she has the wine again.
Continuity mistake: When "Crater Face" comes to the dance with Marty and Rizzo, he takes his sunglasses off twice.
Continuity mistake: In the scene where the whole gang is eating at the soda shop, everybody starts leaving. Doody is in the back, in the middle, on the railing. He's getting ready to leave, takes a bite of his ice cream and sets it down. In the next shot, he sets it down again.
Continuity mistake: In "We go together" they all dance forwards. When the camera changes angle Frenchy and Doody have switched sides.
Continuity mistake: When Marty is talking to Vince Fontaine before the dance contest, towards the end of their conversation she starts to lift up her purse with her right hand. But suddenly it cuts to another angle, and her arm is down.
Continuity mistake: In the middle of the dance contest, Patty Simcox does a funny front walkover, and comes dancing towards the camera. Over her shoulder, you can see Sonny running up behind her. But suddenly it cuts to another angle, and Kenickie is running up behind her to lift up her dress, and Sonny is nowhere to be seen.
Continuity mistake: After Danny has danced around the car with the saran wrap he is shown spinning around numerous times, and then stops with his back to the camera. However, in the very next shot, his arms are suddenly up in a pose far too quickly for him to have done in the time alloted.
Continuity mistake: When Danny is first shown at school, he is speaking face-to-face with a girl in a blue sweater. The immediate next shot, Danny is talking to a totally different girl in a purple sweater, and he has his back turned from the camera.
Answer: Well, Danny was out of town, at the beach. It's quite possible that the rest of T-Birds didn't live close enough to each other to see each other during the summer. Also, they may have all had jobs, or been travelling with their families. It's also possible that they DID see each other, although maybe not as "T-Birds". When they saw each other on the first day of school, though, they were Seniors, and they were ready and excited to be T-Bird seniors.
BGraz
Another possible reason is that not all of them were allowed to see friends during the summer. I had a couple of classmates whose parents were strict about them focusing on schoolwork and/or getting a job. No phone calls or visiting friends on the weekends or during summer: "You can see your friends at school."
Do you really think any of the T-Birds had strict parents?
People quite often rebel against strict parents.
When they asked Kenickie where he was, he said "working, which is more than I can say for any of youse kids" suggesting that the 3 stooges (pun intended for their stupid routine that prompts Danny to tell them to "be cool") didn't work all summer. Also, Sonny needed to borrow money in the dinner until he could get his allowance.
In regards to not living close enough to each other, it is worth mentioning that having access to a vehicle was much less common compared to nowadays.
KeyZOid