Another alternative is charging $10/month, but allowing students or anyone to request a waiving of fees. They just need to take a minimal speed-bump action to request it, e.g., sending an email once per year. Anyone who isn't willing to do this probably isn't getting much value anyway, and a non-trivial fraction of people (more than 1%) would plausibly pay $10/year just to avoid the speed bump or to support the mission.