Sounds like a lie to me. Why not just tell them the truth that you don't want to do anything?
No, it's a mental reservation. "I am off at 10" is a factually true statement. Now, the deciding factor where mental reservation is permitted has to do with whether the person has a right to that information. If the person has nefarious intent, for instance, such as, "hey, let's go to a strip club at 8" and you responded "I am off at 10", that would be perfectly permissible. If your parents ask you to do some chores at 8PM and you say "I am off at 10," then that's a sin because they have a right to know the truth of the matter. There's some ambiguity about when someone has the right to know something.
There's the classic example of would-be murderers seeking an innocent person in your home. Quite a few theologians hold that it is perfectly permissible to say, "[that guy] is not in my house." ... with the withheld completion of "as far as you're permitted to know" or "as far as your concerned." I agree with them. Remaining silent would give the guy away, since they would immediately suspect that the person was there, or saying "I refuse to tell you" ... leading to them perhaps making a thorough search of the property. They have no right to that information since they intend to do evil with it.