Like you, I go on instinct, basing my decisions on th current students' skills, numbers, and needs.
Choosing a play is an art, so a rubric seems such a weird, data driven choice. But, here are questions I ask.
1. How many students are planning to audition? (I work at a small school so I can get a ballpark pretty easily.
2. How many can we easily accommodate? (Based on budget and experience for the students ) I usually do straight plays, with 15-25 people so I like ensemble plays.
3. What are the design requirements and can we ( that is, the current crop of stagecraft kids) accomplish the requirements within our rehearsal schedule and budget? Will it teach them a new skill?
4. Will this play build in skills while also giving students a new challenge?
5. What is the cast breakdown? Types? Male/female/either needs and will I have that cast available? Will it challenge them? And how can I give new actors a chance to shine while also giving new challenges to experienced actors?
6. What are the costume requirements and can we furnish those? How about the set? Costs associated with these items?
7. What are the royalty costs and do I have that in my budget? (Some plays are more expensive than others, requiring not only a set number of scripts but sometimes now also stage manager scripts. )
8. Is the content appropriate for my audience? ( This one is tricky. I've never had anyone question a choice, but I also rejected a play once that was great until the ending which was about incest. No thank you for my audience. I also rejected a very funny play that would have been perfect though the villain was a comical bad German. Our three German teachers all said it would be fine when I shared it with them but were tired of that old trope. They were right .)
9. Assess the rehearsal schedule. Is there time to do this show right?
10. What is the running time? (I usually can't do anything over 2 or 2.15 hours because some of my cast and audience have to catch the last boat to the nearby island where they live.
11. And my favorite. Can I see the play in my head? This, to me, means, can I see a creative way to stage this play. If not, then I don't do it. This holds for plays I've done more than once. Can I see a new way to do it?
Hope this helps! It's general and gives people a sense of the difficulty of choosing a play. I tell people I often read 50 to 100 plats to find my three every year!