Yep, I'm 99.9% sure it's your school's firewall and security protocols that are not letting you in.
If your school IT won't let you into Dropbox, you can always set up a "send to" e-mail address for your Dropbox account. There is a service called "Send to Dropbox" that allows you to create an e-mail inbox within Dropbox for attachments. Once it's set up, you can send pictures (as an attachment) for later viewing and editing.
No, it's not as convenient as having direct access, but maybe it'll do?
Marilyn