GrabCAD does not limit or has any rule for this. Perhaps it might be the school itself not allowing 3rd party signups. Sometimes this is the case with especially school accounts. Here's google's page for more information: https://support.google.com/mail/thread/11362011/this-account-is-not-allowed-to-sign-in-within-the-network-talk-to-network-admin-which-is-me?hl=en
Could you please share the error you're receiving?
The Computer-Aided Design ("CAD") files and all associated content posted to this website are created, uploaded, managed and owned by third-party users. Each CAD and any associated text, image or data is in no way sponsored by or affiliated with any company, organization or real-world item, product, or good it may purport to portray.