Corrected entry: At the beginning you see the coach scouting basketballers when track is going on. That same day the coach is getting the T-Birds ready for football. It's like the school has all three major sports going on at the same time.
Corrected entry: When the T-birds begin singing "Prowlin'" at the gas station, Louis is wearing light grey pants, but when they are shown on the stage he has blue jeans on, while everyone else is still in the same clothes they had on at the gas station.
Correction: So? It is obviously a different day. Just because the other Birds are wearing the same or similar clothes doesn't mean Louis has to as well.
This correction is illogical, why would the T-birds break up a song over the course of two days, and have only one guy change clothes, when every other song in the film is done in "real time"? This entry does not need a correction.
The only one who appears to be in the same clothes is Johnny, wearing black pants and shirt. The others have different shirts. As for the pants, many people wear pants multiple times before they are washed. For Johnny, he likely has a wardrobe that consists heavily of black clothing - to make him look cooler. It could still be a different set of clothing. As for the song, it is sung earlier in the film as well - during the T-Birds' audition. The singing at the station and the stage is practice.
Correction: When Michael is talking to Frenchy by the fence, you can hear him say, "How's your first week been?" It's safe to assume the sports were held on different days.