Visible crew/equipment: In the scene during the American bandstand, at the end of the song when the teacher is dancing in front of the band members, the crews can be seen. The dressing and the hair doesn't match Grease era at all, and someone is sitting on a director's chair as well. (01:06:20)
Visible crew/equipment: In the soda shop scene where Sandy and Danny are at the jukebox, the reflection of the boom mic is clearly visible in the chrome part of the jukebox. [True of videos and early DVD releases - in later DVD releases the reflection has been removed].
Visible crew/equipment: In the scene where they are singing Grease Lighting - the part when the fuzzy dice float to Kenickie - after he takes them "out of the air" and swings them around, then lets them fly - if you look close to his head you can see the white string they were attached to when they were "floating".
Visible crew/equipment: When Danny is about to play basketball with the other students and Coach Calhoun is speaking to him, you can see the shadow of the boom mic behind the coach.
Visible crew/equipment: When Sonny snaps Tom's suspenders as he enters the gym, you can see a microphone in the top right corner.
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