So im using Posca markers for a school art project, but the blue paint from the marker keeps cracking when t dries. Any idea on how to stop it from doing that? Even the re-coats crack.

Don't put so much of the ink down, it cracks because there is to much ink.  So go lightly over what you need to do.  

Explanation:

The more ink you have the longer it takes to dry plus also it will crack because the in shrinks when it drys so put less for less cracking.