My church, Cedar Ridge Community Church, has a 1 acre vegetable garden where we grow food to donate to people in need. The managers of the garden can sign off on volunteer hours for your high schooler. There should be a contact on the website - just email one of the pastoral staff.

Look at this site. They have a section on MD and VA.
https://dcurbangardenernetwork.wordpress.com/

Wondering if anyone knows about a community garden that needs volunteers in Montgomery County, MD? Would they take HS students as volunteers?