Ethics: Having students use my referral link for a product used in the classroom?
There is a saying, "Caesar's wife must be above suspicion," which I believe applies well to an ethical dilemma of this sort.  Despite the somewhat nasty origins of the quote, what it has come to mean is that in some circumstances one must not only act ethically but also act to ensure a very clear appearance of ethical behavior.

In this case, the question is this: could a reasonable person question whether you might have chosen to recommend this service in part because of the benefits that redound to you yourself?

In this case, since there are a number of other ways that you might have gone about obtaining similar capabilities for the class, it would be reasonable for a student or other person to wonder if you were influenced to recommend that particular site due to the benefits that you would obtain through the referrals.  Given your position of authority, then, I believe that does create a genuine ethical concern regarding the referral program.

I see two basic approaches to resolving this:


Recommend the service, but do not make use of the referral program.
Recommend both this service and several of the other competing services.  Note that this is the one that you personally use, and for this service provide both a non-referral and referral link (being transparent about the benefit to you).


Personally, I would prefer the first option, since it's completely unambiguous, but would not find the second particularly problematic.