Hi

Yes, you can study computer science in your graduation even if you  have not studied  computer in your class 11th and class 12th .

The general eligibility criteria for B tech computer science is that you should have completed 10 + 2 with physics , maths and chemistry and some colleges may have criteria of at least 75% in your 12th board examination but it is not mandatory to have computer science in your 10+2 to be eligible for BTech computer science.

For example,

Jee Main is the most widely accepted entrance examination for engineering through which you can get admission in your BTech computer science and other branches of Engineering and is eligibility criteria of JEE mains says that

to be eligible  for JEE mains 2021 for BE or Btech you should be either appearing for 12th board exam or equivalent  in 2021 or should have passed it in 2020 or 2019 with Physics,  mathematics and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject.

For detailed information go through

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

Similarly,

The eligibility criteria BSC Computer Science is that you should have qualification of 10 + 2 with physics chemistry and mathematics and generally a minimum of 45% in your 12 board examination is required to be eligible and generally admission is on the basis of your marks in 12th board examination but yes some  college/ universities like BHU does conduct entrance examination.

You can check the same at

https://www.careers360.com/courses/b-sc-computer-science

In short,

It is not mandatory to have studied computer in your 10 + 2 to be eligible for studying computer science in your graduation

Thank you