How school subject blocks work and how to negotiate them
The rule you cannot see in any CBSE document: the timetable.
What a block is
Schools group subjects that are taught in the same periods. You choose one subject from each block. A combination CBSE allows may be impossible at your school simply because two subjects sit in the same block.
Get the sheet
Ask the school for next year's blocks in writing, ideally before the Class 10 pre-boards. Compare them with the combination you built in the planner.
Negotiating
Schools open new sections when enough students ask. Find classmates who want the same mix and ask together. If the school cannot, decide which subject matters more and whether another school is a serious option.
Guidance based on the CBSE scheme of studies and public exam bulletins. Confirm rules with your school and the official sources before deciding.