ZSE Controller reported flight school crew stated they forgot to fly published missed approach while in IMC resulting in entering a higher MVA and a CFTT event. ZSE Controller stated this is a frequent event.

Narrative

Aircraft X flew the rnav34 to s33 IFR. Instead of flying northbound to gashe; as published; they flew ENE. I tried calling them in the blind and no joy. Eventually I reached them and asked them if they had cancelled IFR and they said they were still IFR. I asked why they weren't flying the published missed approach and they said they forgot. I issued a low altitude alert and told them to 'Join the published missed approach' and climb when they were below the MIA. It is my understanding that joining is at the pilot's discretion; and I didn't want to issue a direct clearance or let them continue heading towards higher terrain away from a flight checked area like the missed approach. I issued a pilot deviation to the pilot.The aircraft was deviating from assigned headings AFTER this issue as I vectored them for their next IFR approach.Every shift it seems like there are pilot deviations in the redmond area. It is one thing if its a solo student; but these are with instructors on board with little to no visibility in the smoke. I don't know how to make these flight school pilots fly their aircraft as cleared by ATC.