Free screen reader found at: http://www.nvaccess.org/download/
They ask for a donation and you might want to consider doing so because NVDA is an excellent alternative to commercial screen readers.
NVDA does everything commercial screen readers do providing voiced access to the computer, desktop, programs, internet, email and Braille displays.
NVDA can be installed to either a computer or to a flash drive. Choose portable installs for both NVDA and Jarte, placing both on a flash drive and your students can use any computer. NVDA provides screen reading capabilities and Jarte takes care of the word processing chores.
If you already have a commercial screen reader, NVDA can reside on the same computer. If your commercial screen reader is set to start-up with the computer. Shut it down and start NVDA. NVDA should work fine.