Audio problem: During "We Go Together" at the end, John Travolta's mouth is not in sync with the words "we'll always be like one." He appears to start singing "that's the way it should be" before realizing and correcting his mistake.
Audio problem: At the cotton candy stand at the graduation carnival, Blanche is seen silently mouthing the question "How many?" In the next scene, from a different camera angle, the same question is asked - this time with audio intact.
Audio problem: When Johnny Casino & the Gamblers perform "Hound Dog" at the National Dance Off, you can hear the song, but if you look carefully you can see it does not match the lead singer's lips.
Audio problem: When Sonny says "Shut up!" to Putzie outside Kenickie's car, his mouth doesn't move.
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.
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