What do you mean by "not very good at school?" If school is really hard for you, even though you try as hard as you can, then trying to become a veterinarian, which is an actual doctor, will be difficult. If you could do better if you tried harder, then, it's a possibility. But, you would have to be self-motivated and really determined. There's not going to be someone standing over you and encouraging you every step of the way.
If it was me. . . I'd aim for the highest possible career choice - veterinarian - knowing that I could always adjust downwards if it becomes clear that I'm in over my head. But, no matter what position I was aiming for, I'd try as hard as I could!
Another possibility is to aim first for vet tech, and, then use that as a stepping stone to get to vet school. A tech at my vet's office did that, and now she works there as a vet! If you do that, the experience that you gain as a tech will help you to understand those things you have to learn at vet school. Please give this option some consideration!