No person, Whether they are female or male, Should be forced to wear a skirt. Some even absolutely hate skirts. It is also forcing trans guys to wear skirts.
That being said, Boys should be allowed to wear a skirt if they wish. It is sexist and demeaning to exclude them.
If a girl want to were trousers journg school that's fine they don't have to were skirts.
If they don't want boys see there underwear or anything like that they don't have to they should just were trousers if they want boys to see there underwear they can't just were skirts but if they want to were trousers they should be able to