Not sure this is working as punishment.

I assume that the lines behind him read "I must not jerk off in class" or something close to that. But dropping his jeans in front of everyone seems to be having the effect of getting him to 'half-mast' and very soon he'll be showing off a full hard-on. I guess that's exactly what the master was hoping for, though of course he wouldn't say it. I'd think there'd be other boys in the class wanting the same treatment.