If you follow the collectible guides, you shouldn't worry as this achievement comes automatically.
Not sure why this is missable. I guess it has to do with the rating of your actions. If they are considered "good" or "bad".
The point of no return for this level is in the big hall in the bottom right of the map. All collectibles for School are before this hall.
When you finish the School level you will have a short sequence somewhere else. This will then unlock:
It is really short and has no colletibles.
Shortly after this the Out of Summer achievement unlocks (my time stamps for these 2 achievements are approx 1 min appart).