Al, first off, yes, you are allowed to modify Steve's patterns, but you're not allowed to post the changed pattern. And, make sure you give Steve credit for his pattern. As for logos, most are copyrighted, so be careful what you do with it. I don't know if high school logos are copyrighted, but it's better to be safe than sorry! As for how to modify it, I'm a cheapskate and use Inkscape. Steve's patterns are in .pdf format and can be imported straight into Inkscape. I don't know if Coreldraw can import a .pdf or not.
Hope this helps!