I agree entirely: what my body can do at morning workouts varies quite a bit from day to day and has a lot to do with what it's been doing (lifting weights, I'm almost always stronger after a rest day, for instance).

I learned a new aspect one day ten years ago while preparing for my first (and to date only) mountain bike race, and this may be pertinent to you since your classes are in the evenings. Normally I rode trails on weekend mornings, and rode on a relatively flat dirt road near home in the mornings before work. But with the race coming up, to get in a bit of extra practice, one day I packed my bike along and did a trail ride one evening after work. I didn't ride well at all that day. My body wasn't tired in the least: I'd done nothing more strenuous than sitting at a desk all day. But mountain biking requires courage and strategy, and my brain was too tired to do either one well.