If he was leaning hard, but not hyper/excited, that does sound like he might have been leaning on you for balance. It's similar to the horse being heavy in your hands while riding. Rather than carrying himself, he's flopping along on the forehand, bracing against something handy to stay balanced. If you're going to be repeating the same exercise in the future, it may be a good idea to use side reins to give him a little contact, and ask him to carry himself. After all the whole idea of the exercise was to convince him to go nicely in canter in that circle, right?
I wonder if he was acting up during the test because of something you changed? Was there something in the lesson that you were finding difficult? Perhaps you were trying really hard and had gotten a little tense.
Lunge lines do some in different lengths. When I have space, I usually start lunge canter circles as large as possible; I'll walk a noticeable circle, following the horse, rather than making her stay at the same fixed distance from me. If she has enough pull on the lunge line to keep her in the circle, but NOT enough to lean on, she will have to carry herself
at which point she could balance more easily on a smaller circle. So it would be easier to shorten the lunge line.
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