Any proxy server that allows public accounts provides this service. Proxy.org lists available proxies, updated every 10 minutes, and VPNs, and provides explanations of proxy and VPN types. Proxy and VPNs services often charge a fee for use.
Be aware of the school’s penalties if you are caught using a blocked site. Federal law requires that schools block harmful sites, and schools interpret this law broadly. Students have been suspended or expelled for accessing blocked sites, even when away from school grounds. While students do sue over this, a lawsuit is not a good fallback plan, and does not always work, particularly if the student used a school laptop or tablet.
Alternatives to using a proxy or VPN exist, but none of the solutions are entirely untraceable. For example, IT can see that someone is accessing the same IP address consistently, find out that the IP address goes to a proxy server, then check who is using that workstation at that time.