Is canceling student loan debt considered stealing?

Eh, maybe. Back during feudalism, emancipation of serfs was also considered theft from the nobles who owned the land (and thus the serfs who worked it).

Sometimes governments implemented programs to reimburse the nobles for losing "their" serfs, and sometimes not. Now that we're a couple centuries removed from that drama, we generally accept that the destruction of feudalism was a good thing, regardless of whether it was theft.