What's the right preposition to use with the verb "enroll"?

I generally hear enrol at an institution, enrol in or on a programme or course, and if you are the mouthpiece of the institution, I hear enrol with us.

Here's Macmillan:

Enrol

VERB [INTRANSITIVE/TRANSITIVE] British English pronunciation: enrol /ɪnˈrəʊl/

if you enrol at a school or other institution, or if someone enrols you there, you put your name on the official list of its students or members

enrol at: Andrew cannot enrol at his local school because the class is full.

enrol someone in/on something: Isabelle enrolled her brother in a training programme for engineers.

From Cambridge Dictionary Online :

Enroll at an institution

Enroll in/on/for a course/department

Enroll with someone

Enroll at (a/an) = college, institute, university, name + school + at + the + school.

Enroll in (a/an) = college, program, course, school, class, exchange program, choir, academy.

"Enrol on" is a British term for enroll in.

Enroll for (a/an) = check, vacancy, course, semester.

The TLTR version is that if you want to be on the safe side, always use the verb with the preposition in.

It can also be used with the other two prepositions but they're not as frequent as the former - a word of caution is that the combination with the preposition on is more common in British English.

Hope that is of help