How do I find percentage from two values which are coming from two different population sets that has large difference in population sizes?

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I need to find percentage from two values which are coming from two different population sets that has large difference in population sizes. They are also different in nature. I set is teachers and another set is students.


I have the performance score of 10 schools.
Each schools have filled out 4 data collection formats.
Each data collection formats are for different reporting units. (first format for students ( e,g. performance 3, if 80% students scored A grade 2 if 60 and 1 if 40 or less,
second format for infrastructure,
third for teachers
forth for extra curricular activities
Number of questions varies in each formats.
Have counted the total score for each format and divided it by maximum score.
Then based on the sore each themes of the school was graded A (80<%), B (60 to 79%) C (40 to 59%), D (20-39%), and E (1 to 19%).

Questions

If Same school got 4 grades in 4 categories, Is it alright if I give a value to grades (A = 5 and B = 4 etc..) and add up 4 different grade to get a composite grade. (5+4+1+3)/20?

I wanted to take one question from format for students and another questions format for teachers and do the same process. Example attitude towards digital learning. teachers and students have answered on a three point scale.
number of teachers 20, number of students 500.
Teachers' average rating is 3 and students' average rating is 2. How do I add these two values together to show a percentage?