At the beginning of the second book it is claimed that they will not know if the cleansing worked or the seniors survived until they themselves get out, but it isn't true. Graduation is in the morning, induction just before dinner time, so the enclave freshmen will know whether their seniors made it out.
We get the size of the gym from the statement that from one end of the gym to the other and back is about 150 meters, the same as the distance they will have to run to get out at graduation. That makes the length of the gym 75 meters. If we assume the width is as great as the length — if it were longer it would be the length — that gives an area of 5625 square meters. At field day all the students are in the gym, and there are about 5000 of them. So they have at most a little more than one square meter each, even without allowing space for the food machines and gym apparatus. That is not consistent with the description of field day in the books.
About a quarter of the students in an average year survive the Scholomance, which means a bit under 400 students graduating. We are told that for each place there are two applicants, which suggests about as many students not getting in as getting in, possibly more, and the ones outside have a survival rate of one in twenty. With equal numbers in and out that means that out of 3000 14 year olds, fewer than 500 survive. Some wizard children, such as Aadhya's sister, die before they get to 14. So the fraction of children surviving to adulthood is at most one in six, probably lower.
It follows that in order to maintain the population, the average couple has to have at least twelve children, probably more. But we are told in the third book that the wizard population is increasing, so it should be more than that. No family we see described in the books has anything close to twelve children — I think Liu's grandmother had six, most families mentioned have fewer than that including two cases (El and Orion) of couples with only one child.
For a somewhat less clear case, it is repeatedly claimed, near the end of the second book, that they have no way of keeping the cleansing going in the long run. Claiming that is necessary for Novik's plot but there does not seem to be any good reason for the students to believe it. Protecting the machinery for a couple of years only requires wrapping it in tinfoil. The graduation hall will be almost empty as long as the cleansing is working — El at this point has seen that it is empty now. So one of the graduating teams could detour to the cleansing machinery, rewrap it, and kill any visible aglos.
The counter-argument is supposed to be that another mawmouth will show up. But that mawmouth will be by the gates, for the same reasons the two big ones will be, so won't keep them from rewrapping the machinery. It might grab a few students on their way out — or might not, given that it has just been blinded and had its tentacles burned off. But even if it did, from the standpoint of students used to a mortality rate on graduation of over fifty percent, getting it down to losing a few students is a win — especially since, with the cleansing working in the graduation hall as well as the rest of the school, there are going to be a lot fewer mals in the school.
Any mawmouths that get in will be considerably smaller than Patience and Fortitude. Since those were substantially injured by one dose of flame it is unlikely that smaller ones will survive two doses a year for very long — or even choose to stay in the school, given that they can leave.
All of this is in terms of what the students know at that point, so the claim that anyone who thinks it can be done can be obviously shown to be wrong is not supportable.
My view is that Novik is brilliantly good at designing the emotional structure of the stories but doesn't pay much attention to the physical consistency of her worldbuilding.