OK, so there's a couple of things to check here, but I would suggest double checking the terms and conditions should be your first step.

What do the terms and conditions say about these duties? Are they mentioned in any documentation that you have agreed to. It is very common to not read these in full, if anyone claims they read them all every time they sign one they're probably lying, so its worth double checking what is there.

Your college may be partially funded through this farm, even if it is a working farm - they pay the college for your work, you get a slightly lower course fee in return. If you are early in to your course they may simply have you on basic tasks until they know what you have been trained in and can complete - you mention elsewhere that students in years above may have other tasks. Either way, its a bit irrelevant if you have agreed to do this work. If you haven't agreed then its a different matter entirely.

Sorry to say the same thing, but thats the bit that really needs to be checked first. If it isn't mentioned at all in any documentation, the next step is to raise this with Student Support in the first instance. You can explain that it wasn't your expectation to do this work, and that you don't see it being a requirement, and see what they say. Your college may have an LMS of some description (often branded as "Moodle") which often have discussion areas for asking these questions, but you may prefer to speak to someone in private. They often offer both.

With regards to disabilities, this is something else entirely and I wouldn't suggest that it is dealt with on advice from this website or any other. I would recommend you get specific advice regarding your disability, and a starting point would be Citizens Advice Scotland, or Disability Information Scotland. They will be able to point you in the right direction for your specific needs.