This is the third part of a trilogy and as such the novelty of the scenario is fading. The first movie was surprisingly good and not as expected. The second movie played in an entirely different environment which added a few new elements to the story. Now the third movie's setting is back where it all started: at school.
To me it seemed like the movie does not really know what it wants to do. In the first two movies, the main protagonist (the ex-con teacher) gradually opened up to the children, became friendlier, and started to care for them. This movie reboots the whole thing by making our guy dislike the children at first, only to let him warm up throughout the movie once more. In general the movie seems to re-establish many things for viewers who have not seen the first two movies. The same thing goes for the jokes, which are very similar to the two previous movies, too similar even. Overall I just felt like the two movies never happened and we are seeing more or less the same thing again. Maybe this is why the first half-hour of the movie also seemed a bit like not having a clear purpose at all. The movie wants to establish the situation, but not really. It wants to continue where the predecessors left off, but not really. It throws around jokes that are not new, really.
The second part of the movie gets more interesting and starts following a clearer story-line, albeit in a linear and predictable way. This is not unsurprising for a comedy, but compared to the predecessors it is just not as good.
If you truly enjoyed the first two movies, you will still like this one. If you did not care for the first two movies, this one is definitely not for you. It suffers from exactly the same problems as similar trilogies at the point of the third installment. Take Despicable Me for example. Not a bad movie, just nothing new and no surprises for the viewer.