Thoughts on youth 12 through 18 teaching primary on 5th Sundays?

Here's how I would approach this:

A teacher who teaches monthly or quarterly is teaching on a regular basis and should be called to that position. If ward members are used to teach regularly in this way, they should be called. I think youth teaching Primary could be (and is) done effectively on rare occasion, but quarterly is too frequent in my view. If the point of having the youth teach is to allow all teachers to attend fifth Sunday meetings, the Bishopric should be taking care of the youth and Primary while all others attend fifth Sunday meetings. Otherwise, the intent of having youth teach Primary is not being met because, instead, workload is doubling for two auxiliary presidencies (probably 8-12 leaders PLUS 25-50 youth) so that a small group of teachers (12 maximum) can attend one 50-minute meeting.

For your information, I've been asked this question before, and there is no directive in the new handbook for this situation. Any handbook directive handles calling youth permanently to teach Primary.

The correct way to handle your concern is to address the issue first with your ward council. It is very likely that others are seeing the same problems you are. This is very likely to address your concerns, but if not, call your stake Primary and/or stake YW president for direction.

You should know that the new Come, Follow Me manual does encourage classes to prepare lessons and teach them to others:

Teach the lesson
Invite a member of the class or quorum to teach part of the lesson.

Teach another class
Ask the youth to prepare a brief lesson about the doctrine and teach it to another class (as approved by the bishop). [https://www.lds.org/youth/learn/learning-teaching-ideas/apyw/learn-together?lang=eng]

You could have your students take an opportunity to teach a simple doctrine to a Primary class, however, you would need to do your lesson preparation at Mutual or during Sunday classes, and then coordinate with a Primary teacher to teach. It would be best to have students teach individual Primary classes as opposed to sharing time, since these kids are learning how to teach. Probably better not throw them in at the deep end. :)

It seems pretty clear from Come Follow Me that teaching other classes or teaching a quorum is intended to be a gradual learning experience rather than a full on PrimaryTakeOver. Leaders should use judgment by helping students practice teaching a portion of a lesson within a quorum meeting first, then perhaps a fuller lesson in mutual, and then progress to teaching a portion of a lesson outside of their own quorum but probably not outside the auxiliary, and then PERHAPS graduate to teaching a full lesson in a regular class situation.