Emkana, sorry this has gone belly up! Usually someone posts a 'my child's teacher wrote X' (insert grammatical solecism of choice) and everyone agrees how shocking it is and we all go home happy.
FWIW, I don't think it matters very much when the children are tiny -- and Xenia, my dnephew has come home from his v. expensive pre-prep school with entirely comparable howlers in the teacher's/ assistant's writing. I think it does matter (a) if they are old enough to notice and/or copy the mistake and (b) if the teacher is not prepared to admit that he or she has made a mistake.
So "Oh yes, clever you for spotting the deliberate mistake, I was just checking you were all paying attention, ha ha" is an okay response, but defensive responses, either denying there is a problem or tearing a strip off the child for daring to criticise the teacher is not okay.
And yes, we had endless instances of this, ranging from a standoff between dd1 and her Y2 teacher over the spelling of 'balloon' (teacher though one l, dd1 thought 2), dd1 correcting her Y5 teacher over the difference between a tetrahedron and a square-based pyramid, and the infamous 'gateau-gate' incident that somebody referred to earlier, where dd1 spelled the plural as 'gateaux' and the teacher 'corrected' it to 'gateaus'. This is not a great scenario, since it results in cynical hacked-off children losing respect for the teacher.
Making mistakes is fine (as long as the person has a level of competence appropriate to the job), it's how teachers deal with their mistakes that matters.