This is a great question! I'd say there are lots of opportunities. One place to start is with a Church/Worship place as many have volunteer opportunities available through their youth groups. YMCAs and Jewish Community Centers will often have similar service programs as part of their youth/teen programs.
For high school aged students, I would recommend seeing if there is a service club at their high school. This might include the Interact (Junior Rotary) Club or a Key Club (affiliated with Kiwanis).
If none of these service groups are available (or appealing) there are some other opportunities -- they could simply contact an elderly or infirm neighbor/friend and offer to help them, but they could also contact a local soup kitchen, food pantry, Church, nursing home, community center, or pre-school and offer to volunteer there.
Hope this helps! And they should definitely make sure to document their hours! Looks great on a college resume someday :)