Can I use Microsoft Security Essentials on my school devices? Yes, but you will be breaking Microsoft’s EULA
Is this scenario supported? No
Does Microsoft want you to? No
Do I recommend it from a technical point of view? Yes, if the devices spend more than 10% of their lives away from the school premises and you have taught pupils to manage the anti-virus updates.
The restrictions are a little confusing but, essentially, free usage is restricted to home users. The EULA states:
“You may install and use any number of copies of the software on your devices in your household for use by people who reside there or for use in your home-based small business”
Use outside of the home is prohibited so businesses, schools, charities etc cannot use MSE for Anti-Virus.
For commercial AV & protection, Microsoft offer the Forefront family of products.