Other schools use Klasku too — can our data get mixed with theirs?

No. Each school occupies its own data space, and the separation is enforced in the database layer: every read carries the identity of the signed-in user, so rows belonging to another school are never returned.

That means a cross-school leak is not something a single screen that forgot to filter can produce — the filter is not in the screen. Students, classes, invoices, messages, and reports all follow the same boundary.