The overall aim of ocarina, from my perspective, is joyful musicing in groups and individually. Each ocarina session should last 15 to 20 minutes and allow each student to play as much as possible. It is important to review (for enjoyment) songs that have already been learned. In other words, children should be allowed to play what they know. I feel that at least half of the time should be spent playing stuff with which the children are already familiar.
To start, give the children the ocarinas in the boxes and ask them to NOT take them out yet. Demonstrate how to hold the ocarina and how to play C. Tell them that they are going to be allowed to play it after agreeing on some basic guidelines. Teach them a specific signal that will be given when you would like to have them stop playing. For example, I hold five fingers in the air and drop them one by one. When the last finger comes down and my hand comes down, that is time to stop. I call it "taking five" to play. Other signals (like saying "Stop") could work as well or in addition. I probably use this one the most. The important thing is to be consistent and practice the signals. Do not let them play out of turn. If you do, that will signal to them that it is okay to play out of turn.
Next, invite the students to take the ocarina out of the box and practice playing C. Encourage them to cover all of the holes. Walk around the room and make sure everyone can play C. Give the signal to stop. Practice stopping if needed. Let them try again, and again, until everyone can play C. It might be appropriate to let individual students demonstrate C. Sometimes, the children play either too loud or too soft (usually too soft on ocarina, too loud on recorder, in my experience). You need a solid and steady air stream to bring the note up to pitch. Ocarinas are much more difficult to over blow than recorders.
Once everyone has C figured out, move on to D and E. If you feel comfortable doing so, play, sing, or say a rhythmic pattern using one of the notes and have the students echo back. Keeping giving patterns with one or more notes. Make sure there are no pauses after they have echoed the pattern and before the next one begins (this down time provides opportunities for goofing off).
Now that they have figured out C, D, and E, practice improvising using the following video available on youtube: http://www.youtube.com/watch?v=hQtZh4d0zsI&feature=youtu.be.
Next, the children will likely be ready to play Hot Cross Buns. Have them watch the youtube video: http://www.youtube.com/watch?v=hQtZh4d0zsI&feature=youtu.be and sing along with the song.
Then have them sing the notes and show where the fingers go for each one (sing and show). Then let the students play the song. Chances are they will have some difficulty starting each pitch, so talk to them about using their tongue; each note begins with a "too" sound with the air. Practice echoing patterns on C using "too".