Hanging up blackboard adhesive over a rough textured wall
A $15 bucket of drywall compound (mud) and a wide drywall knife, a wall sanding block and some drywall sanding screens. Even as a temporary fix, unless you want to hang up a chalkboard as suggested in the comments, the skim-coat of drywall mud will get the job done as quickly and easily as it can be done. If you'll be doing the work in 3 months, it will also provide hands-on practice that you don't have to care about being "perfect" if you'll be ripping and replacing in 3 months. As suggested in that thread, try sanding off the high points before you start mudding.

Big hint if you haven't done much - do not try to get it perfect in one coat. The secret to quick and easy mudwork is to let it dry, knock off the high points, and recoat to fill the low spots, repeatedly.