why is it not "shule"?
The -ni makes school a location, so whenever you have "at" school, "in" school, "to" school, etc., you should usually use -ni.
He has not gone to school.
The teacher has not gone to school.
Mwalimu hajaenda shule.
I think both version of shule and shuleni are ok and thus should be accepted. The way it is now, it doesn't make sense. I reported both
But in an earlier example "shuleni" was not accepted for "to school." Make up your minds!