Just like in English, you can use the present -na- (like the present continuous is/am/are ...-ing) to talk about future actions if it's clear from context that it's about the future. The kesho in this sentence makes it completely clear.
"mwalimu wa darasa atakuja kesho" is noted as incorrect, with a typo. It should apparently be "anakuja". I realise that anakuja might be common but surely atakuja is the more correct version. Can anyone shed light on this?