The problem is this was done is a public school, funded by taxpayer money. Although most of its population are muslim, Indonesia is not an islamic country (except Aceh, which is a special region with its own laws and autonomous government), so the public schools must accommodate every religion recognized by the country (there are 6 of them). If it's happening in a private school (e.g. islamic boarding schools or catholic schools), no one would bat an eye.

Why is it ok to force your kid to eat breakfast in the morning but not ok to force them to do <insert horrible thing>.

Why is it ok to force kids to remove their hijabs but not ok to force them to do <insert horrible thing>.

https://en.wikipedia.org/wiki/Islamic_scarf_controversy_in_F...

you've presumed that forcing kids to a dress code is horrible. it isn't horrible. it's just that you (seem to not) value modesty in dress, which they do.