Are mentally disabled students capable of lying?

Depends on the extent of disability. If he or she can manage to express thoughts or desire, they can probably lie.

Yes, even a small child with developmental issues can lie. But in the case you are talking about, it is not serious or malicious.

What you are not listening to is this person telling you how he feels about you. This can be a problem and you need to let the school counsellor know as well as his teacher. He does not have the same thinking processes required to be able to recognise you do not feel the same about him.
As far as his saying your 'crush' has a girlfriend he may be saying that as he wants you to be his girlfriend. Lying as we know it is not the same with developmentally challenged people. He could be copying what someone else has said. The fact he asks you not to ring your 'crush' as "they are busy" is a big clue for you to give your time to him.
Please see someone as soon as possible.

I have a grandson with mild mental retardation, learning disabilities. He will occasionally lie, but he doesn't seem to really understand what lieing is.
***

Yes, they are capable of lying. They are just like everyone else.

I think that if he's capable of having feelings for you, he's capable of lying...

maybe.

lie or not lie is not important , the point that are you still love him, he lie means he feels you are vip in his life . i support you fell love in him. good luck.