I'm trying to figure out the mode and modal class and the numbers that are the mode are 17, 20, 24, 25, 31, 33 from a set of numbers. (I already figured that part out). My question is what of those numbers is the modal class or in other words what is a modal class as opposed to a mode?

Also, which is most appropriate..the mode or modal class to use in an article about average class sizes?

From your set of numbers, there is no mode or modal class. If the scores were grouped into intervals, one could be the modal class.

The mean or median would be better to indicate average class sizes. Any of the measures of central tendency can be used, if the distribution is relatively normal. If the distribution is skewed, the median is best. If the distribution is bimodal, this indicates that possibly there are two underlying distributions, which are best indicated by the modes or modal classes.

I hope this helps.