[hexexom]

Look into OWASP top 10 vulnerabilities, and study them. Download something called Damn Vulnerable Web Application (DVWA) and practice what you learned.

You may also want to remove the school's URL from the forum post. If you do manage the hack the website, this leaves a paper trail for investigators.

SeriouslyToaster_68 gave some good advise.

Also, if you go to this school, I would recommend gaining access to the internal network first. Locate the server on their intranet and scan for vulnerabilities that way. You can also find other vulnerable PC's / servers and then start trying to escalate privileges to find somebody with admin access to their web servers.