Vaskivuoren Yleis- Musiikki- & Tanssilukio

In the 2nd floor, newer part of the building (language classes). Enter through myyrmäenraitti (Entrance C), take a turn to the right, walk through the hallway, then through the glass-walled hallway, go up the stairs- the language classes lobby/hallway is to your left as you walk up the last stairs, so turn left, walk through the entire language hallway with the sofas and benches, and there are two single stall bathrooms, both labelled "inva/henkilökunta" (sadly, the school elevator is not in use for the public and therefore these bathrooms are not accessible for people in wheelchairs) which means "disabled/staff" and you can use either one. Both are unisex.

You need to look like you're a student aged 16-19, or actually be a student. Otherwise you may get kicked out by the teachers. (The language teachers' faculty room is very close to the bathrooms.)