Hello aspirant,

Hope you are doing absolutely great.

So with regard to your query, generally you can attempt most of the entrance exams only after completion of your 12th standard. However, there are some exams which you can attempt after completion of 11th, these are as follows:

1. International Mathematics Olympiad i.e. IMO

2. NTSE

3. KVPY

4. National Science Olympiad i.e. NSO

5. MTSE

Hope this helps you!!