El Shaddai is a British-founded charity that aids impoverished and homeless children throughout Goa; it has a number of schools in Assagao. Volunteers who are able to commit to more than four weeks’ work with El Shaddai can apply through the website. There's a rigorous vetting process, so start well in advance. You can also help with donations and fundraising activities.
For more information, there's a stall at the Anjuna flea market.