Some schools will have imposed restrictions on their network via a Firewall - sometimes, this can be prevent MarvellousMe from working correctly.
In order to remedy this, the school's IT will need to whitelist the following URLs:
https://*.marvellousme.com/
*.marvellousme.com
https://admin.marvellousme.com/
https://teacher.marvellousme.com/
Updated