Noun clause structure "student enrolling"

There was not as many students enrolled in A as in B

Is this sentence correct?

Why it is "students enrolled", not "students enrolling"?

Solution 1:

The use depends on the point in time that the statement is made - during or after the enrollment period.

In the case that you give, one would infer that the class, or course, was already under way and the students had already enrolled.

If the sign up period for the class was still active, then one would use enrolling.

Solution 2:

(1) equals to (1a) in meaning, whereas (2) equals to (2a) in meaning.

(1) There were not as many students enrolled in A as in B.

(1a) Not as many students were enrolled in A as in B.

(2) There were not as many students enrolling in A as in B.

(2a) Not as many students were enrolling in A as in B.