I am studying 12th science(CBSE) in Indian Language School in Nigeria, how can I get admission in B.Sc physics in St Xavier College, Kolkata

It's difficult but in some colleges you may can get 

Yes, you can pursue VFX after completing your 12th grade. You need to choose the right subjects to develop foundation as a VFX artist.

A gap of 1 to 2 years after 12th can be considered normal. Although the students should be able to address it via the valid documents. The reasons like health, finance, skill development or family are accepted. 

And if the gap after the high school Diploma is beyond 2 years, strong documentation

Yes, it should not be hard for you to get admission to the University of Western Australia as an Indian student with 50% in class 12th. However, please note that it does not put you in the top applicants either. The general admission percentage requirement for international students is 50 to 65%.

Loo

Top universities require a minimum of 80-90% in your 12th-grade board exams.

• Average universities often accept students with 60-75%.
• For highly competitive courses like Medicine, cut off is usually higher. 
• Strong SAT/ACT scores can also compensate for a slightly lower board percentage.