There has resently been a event where the alarm has gone off at my school. when we exited out of the building we  encountered that the gates, that lay on the main route to the assembly points, where locked. Is this allowed or something that may be let slide 

Depends on whether everyone would be stuck or there's an alternative, if being stuck or passing down the alternative would be a risk from the fire, if the alternative can take the number of people without bottle-necking or being too slow, if being stuck is a crush hazard as the area is too small etc.

Many factors will influence if this is a trivial or major issue. The competent person responsible for the school's fire risk assessment should be consulted.

From the content of the first post, I cannot tell if there was no way to evacuate or if there was just an issue evacuating quickly because the gate needed to be unlocked.

If they were not able to evacuate, I would think that would be a very big issue.