Board Thread:General Discussion/@comment-108.209.116.49-20180107181300/@comment-26397825-20180112064539

He let him in because the writers decided to let him in. There doesn't have to be an in universe reason, as much as we want there to be - it wouldn't be the first plothole.

Plus, by the way Glynda was griping about it, it seems she was the one who reviewed his transcripts and sent the acceptance letter. She is second in charge after all, and it is a task that can be delegated, which is what any smart head of anything does.