1.

One disadvantage of an overcrowded school is that there is not enough space for the students to play. Healthcare service, the library or the canteen may face problems too.

2.

One disadvantage of an overcrowded bus is that there aren't enough seat for people. The noise is also a problem. It's very uncomfortable to be in a crowded bus. Moreover, crime has more chance to happen.

3.

One disadvantage of an overcrowded block of flats is that there won't be enough space to play or space for collective activities. Moreover, noise is a problem and the air may not be fresh.