To mark a coin for Quests Only, simply locate the Coin in your Admin view's Circulating Coins list. Toggle the Quests Only to the on position. When toggled on, teachers can't manually send the coin. It must be earned in a Quest, from the Send Coin node.
In this example the Haiku Poet coin will not appear in the teachers' coins list. It can only be issued from the Send Coin node in a Quest.