Asarigawa Onsen is a well known hot springs resort between Otaru and Sapporo. It is particularly popular with Hokkaido residents due to the ski fields nearby and with visitors to Otaru.
There are a large number on onsens here, most of them part of somewhat expensive resorts but in natural and quiet settings. Most of the ryokan and hotel onsens allow “higaeri” for around 1000 yen. As it is only 30 minutes by train (530 yen, no reservation required) from Sapporo Station this is one of the more affordable onsen options for students. Asari station has detailed information on available onsens.