IMHO, If you're training for racing, you're better off doing a program of structured intervals on a trainer, or even on a spin bike, rather than a spin class.

In my experience, most spin classes are run by instructors that know little about riding a bike on the road, and nothing about racing one.

And while many spin classes can be hard, they're not designed to work on particular things in a progressive fashion.

If you can find a series of spin classes that are intended for race training that would be different.

Failing that, doing some general spin classes won't hurt, and may help with motiviation, but don't let them get in the way of doing the structured work you need to do, or recovering on easy days so you can do that structured work.