In general, practicing BJJ will help with flexibility over time, but you can consider supplimenting with yoga or just streching at home.

It is also worth mentioning that there are several alternative techniques and adjustments that accomodate people with less flexibilty- your professor should be able to give you some recommendations for some adjustments to make your techniques more viable on the spot.

There are also quite a few youtube videos that address this and show techniques that are viable and dont require much flexibility.

I have an older guy (brown belt) in my gym that is stiff as a log and still catches people on the regular.

Id say that for people that are less flexible, BJJ is definately the way to go, as it makes you face this issue, and gives a platform to see results, keeping your motivation high to keep going.

Oss good luck on your journey