For some reason DL doesn't accept discipulus. Interesting that they allow women wives and men husbands, but it seems that studying is only for girls. I'll report.
why not "Discipulus in forum est"?
in with the accusative implies motion, in forum -> into the market, to the market
e.g. discipulus in forum it -> The student goes into/to the market.
in with the ablative implies location, in foro -> in the market, at the market
Why not "at the market ". It sounds better to me ,but I don't know if a different Latin construction would be needed