It's not a "belief". It's a well-tested and supported scientific theory. A scientific theory is body of knowledge that is supported by the available evidence and which makes testable predictions (which check out when tested).
To pass an exam on big bang cosmology, believing / accepting big bang cosmology is not at all a requirement.
What is required, is for you to demonstrate to have a good understanding of what the theory actually says, how it can be tested, etc.
Your acceptance of that model as being "true" or lack thereof, is not relevant.
Having said that...
The acceptance of big bang cosmology by the scientific community is the result of the evidence in support of it, not of what the school textbooks say.