I understand your position, but giving someone a reward to post a review will strongly bias them in your favor. This has been very well known, for a long time (there are many examples going back decades, Amazon Vine for one.)

The thing is, reviews (in general) are usually biased towards the positive. Even more strongly so when you consider that (in the case of a coding bootcamp) you've just spent an enormous amount of time, money and energy there - nobody's going to want to trash it.

I think we'd agree that in the case where someone had a great experience with your school, they'd post a great review, and when someone had a truly terrible experience, they'd probably post a terrible review. But what about when someone has a merely "meh" experience? In that case, someone will likely (with a bit of a push towards the positive) write a simple, positive review. Maybe they thought they'd have learned faster by themselves... but there's a gift card? And my friends will see the review? Hmm... I'll just write something short and positive.

In general, I think there are a lot of things that are a careful balance, easy to tip in one way or another - politics, public opinion, social issues - and reviews are certainly one of these things.