Continuity mistake: When Jan is eating her lunch, she is joined by Marty and Rizzo. Jan then says "Did you guys get a look at Zuko this morning?" and you can see she has nothing in her right hand, but in the next shot she is holding a fork.
Continuity mistake: During the race at Thunder Road, look at the length of the wheel cutters - they keep changing length throughout the whole race. Plus we only see one come off, but before the crash the two Westside ones are missing.
Audio problem: At the very beginning when they are singing "Summer Loving" Sandy sat at the top of the seating for the athletics track. The voice-over and Sandy's lip movements do not match.
Continuity mistake: When Danny and Sandy enter the sports hall for the National Dance Off, there are four people behind them sitting on the spectator benches, but when Doody and Frenchy enter the hall in the next shot, the people have disappeared and there is now a woman in a pink dress behind the benches.
Continuity mistake: The scene where Sandy and Danny are eating together, then the rest of the gang comes, Sonny walks Marty home. Right after the two leave, Sonny come up behind Frenchie, like he was in the bathroom or at the jukebox. Wasn't he walking Marty home?
Continuity mistake: When Jan is mimicking the Ipana squirrel on TV, she put her hand up to her mouth when she is singing, 'Brush-a Brush-a Brush-a', and pretending to brush her teeth. In the second half of the song, however, after the second line; 'New Ipana toothpaste', she has both her arms pointing in front of her making a 45 degree angle with her body, and her neck straight but in the next shot, which is at the beginning of the next line, 'Brush-a Brush-a Brush-a', she has one hand up to her mouth again and her head is pointed a little to the right. You might also notice that the time between those two lines is longer than it should be.
Continuity mistake: When Sandy tells Danny that he is a fake at the pep rally, we see Rizzo, Frenchie, Jan and Marty standing behind her in a line. In the next shot, Jan is standing directly behind Rizzo.
Continuity mistake: When Tom and Sandy are eating an ice-cream sundae at the ice-cream parlour, Tom gives Sandy some money for the jukebox. After Sandy gets up from her chair, Tom then sits back in his chair and he is looking forward, but in the next shot he is looking over his shoulder at Sandy.
Continuity mistake: When Sandy and Tom are eating an ice-cream sundae at the Frosty Palace, Tom takes a piece of ice-cream and puts the spoon in his mouth, but in the next shot he has his spoon back in the sundae glass.
Continuity mistake: After the race in the LA River, the black losing car is left in the middle of the river. When Sandy is talking to Frenchie, the car has been removed from the river. And finally, it's back in the water again.
Factual error: In the dancehall, the guitarist in the band is using a silverface Fender guitar amp. Fender did not manufacture this style till early-mid 60's. (02:00:00)
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