It means that if there are the same number of students in July as there are now (or fewer) the college will shut down. The sentence is inverted. It is easier to understand if you turn it around: "if enrollments have not increased by next summer, the college will close its doors." That is a standard if/then such as you see in your critical reasoning section.

Thanks for your reply.

The above statement is making a prediction basis certain condition. Is it correct to use present perfect in 'if' clause and 'will' in then clause.

I only knew of instances where present tense is used in 'if' clause and 'will' in then clause to state a prediction.

I think it is important to understand that in SC, an answer could be grammatically correct and still be the incorrect answer for several other reasons.

Discussing a sentence like this one might not be the best way to beat SC.