SPARQL query to get all the "leaf" classes / lowest level classes

I feel like I'm missing something rather obvious. I would like to query my graph so that I get all the "leafs", so any class that does not have a subclass. Basically, I need the inverse of rdfs:hasSubClass (I tried rdfs:superClassOf but it doesn't not exist ahaha).

For example, when I query for the lowest level of the classes by using WHERE {?level1 subClassOf ?level2 ?level2 subClassOf ?level3 ?level3 subClassOf ?level4 etc.}

I eventually end up with kind of what I want: some of the leafs. But if I then join this result to the leafs of one level higher, I also receive the parent classes of the leafs of the query above, which I do not want. I only want children classes, no parents, if that makes sense.

The goal is to be able to output a list of classes that can be instantiated.

Looking for any tips in the right direction! Thank you!