I think Kythcat is correct. It's all about timing. That said, have you checked out business supply stores like Staples? Or drugstores like Walgreen's? Or all-purpose stores like Target? Those are the places I can usually find school glue (and other school supplies) off-season.
If you're still unable to find school glue, there might be other options, depending on what you're trying to accomplish. Is there a glue-stick available that washes out as easily as school glue? An adhesive spray like 505 might meet your needs. A super-thick (100%?) solution of fabric starch might also work - but I've never tried this so don't know.
All that said, I'll bet the brand you purchased will work just fine!
Good luck!