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)
Factual error: The movie and events take place in 1958. However, two Fender amplifiers that are used by the band, "Johnny Casino and The Gamblers" are "Silverface" models that were only manufactured between 1967 and 1981.
Factual error: At the graduation carnival at the end of the movie, Danny enters wearing a letterman's sweater and explains to his buddies that he earned it by lettering in track. However, on his left sleeve are two stripes, indicating that he lettered in two separate years. Since you are only awarded a stripe for each year you letter, and since Danny was a senior when he lettered in track, his Rydell letterman's sweater should have had only one red stripe on his left arm.
Factual error: When the Pink Ladies are walking Sandy into the outdoor cafeteria, just after she tells them the name of the boy she met last summer, all the cars in the background are from the 70's, not from the late 50's or early 60's. This actually happens twice in the same scene.
Factual error: In the scene where Danny is talking to Sandy at the Frosty Palace, the jukebox behind them is a 1973 model, but the film takes place in 1959.
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