It sounds very much like the connection is timing out before the images have been uploaded. To remedy this, contact your school network administrator to check this on the network, you may also need to contact your broadband supplier. This will explain why it works outside of the school network.
A possible workaround could be to upload images in a compressed zip file to reduce the upload time. Once the zip file is uploaded, click on the cogs and then select the zip file. You will have an option to unzip the file.
If you still experience problems, please let the support team at editure know.