So if my understanding is right then its spelt like this
Class's . The additional s is added because the s before it is pronounced, so another is required after the apostrophe to show the change in sound.
The exception to this rule is if the s before the apostrophe is silent.
You also dont need to add the additional s if the word used has the same sound as it does in plural and verbally....for instance boys. You'd spell it as boys' not boys's
Its all about common sense to how something is pronounced.