Can teachers see students' invoices and amounts?

No. The Teachers Portal is deliberately narrow: their own teaching schedule, attendance for the classes they teach, and report cards for those students. Invoices, amounts, balances, and other classes' students are not there.

That boundary is enforced in the data layer rather than hidden from a menu, so a class that is not theirs does not appear even if its address is guessed. Money stays in the tenant panel.