I am a former high-school teacher, and I authored
Learning Stencyl 3.x Game Development: Beginner's Guide.
I used to teach game development to 11-14 year old students, but that was before I knew about Stencyl. If I was teaching high-school students now, I would use Stencyl without hesitation. For those without teaching experience, and without knowledge of the ability of the students, it's tempting to suggest a 'professional' game development system such as Unity, but I am familiar with the classroom environment, and I don't think you would have a chance of success with the hours that you have available. Putting aside the argument about whether or not Stencyl is a 'professional' product, it is not necessary to teach high-school students to use the most popular or professional products; it is important to teach them how to learn. That is, once they have learned the principle of game development, they can then learn to use whichever utilities they want or need to.
Stencyl is suitable for all abilities from lowest to highest; the students will see results quickly, which will give satisfaction and motivation whilst learning the important principles. Education is not about teaching students to learn specific products.
If you have not taught game development previously, then my tips are:
- Ensure that the students do not have unrealistic expectations; show them the kind of games they can expect to create in the allowed timescale.
- Don't allow them to spend much time on asset creation, and preferably provide some assets for them; they can change to their own assets if they have time towards the end of the course.
Have you seen the Stencyl Education Toolkit? It's available from the Education link at the top of the page.