For this iMac, you’ll need an SSD with SATA II fallback support - the pre-2011 models do not support SATA III. In addition, you’ll also need a 2.5>3.5” SSD caddy. Once you mount the drive in the machine, tape the temperature sensor to the drive - not the adapter. Just make sure you check with the school if the maximum version of MacOS this can take (10.11 W/2GB RAM) will work for you. Tell them it’s an old machine you used to use regularly and want to make sure so they know why.
Refer to this guide to see how to change the drive. As long as the SSD supports SATA II you’re good - SATA I compatibility is optional.
If it's pretty much just a situation where they'll be issuing G Suite logins it may not be an issue but if there's local applications involved, then you may need a newer Mac. Even if you find it's too old to use, it's still a good learning experience for your son and it isn't completely useless - he can still use it for things like word processing.
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