My daughter competed for her high school last year and really enjoyed it. She is at a higher level in club gymnastics, but it was actually challenging having to do some skills she hasn't done in a while. Vault in particular can be a hard transition. She did it just for fun and it was very low stress and a good environment. Plus she got to meet many gymnasts from other gyms and enjoyed that. She only had one teammate from her school though, so they weren't able to be competitive as a team which was fine.
It definitely depends on the state and area in general as far as what skills/level they compete at and what the overall environment is like. In my area (central NC), high school gymnastics is at the Xcel Platinum level and is pretty laid back I would say. Only 2 short practices a week and they aren't even mandatory. Most, but not all, of the gymnasts were still competing club gymnastics at the same time, so practice was more about tweaking your routines or getting feedback for them. It does seem like a disadvantage to those not in club gymnastics though.. not sure how they had enough practice time really.