Sleep in regular clothes
Most of the people sleep in the night suits. However, if you want to avoid washing up on a Saturday morning, sleep in the regular clothes. In the morning, it will give an impression to the parents that you have already washed up.
Brush your teeth properly before going to bed; otherwise, you will not feel fresh in the morning and you will not be able to avoid washing up in such a scenario.
Watch TV after breakfast
Try to have a heavy breakfast on a Saturday morning, so that you can delay the lunch. If anyone asks you to wash up, you have an excuse that you will take bath after lunch. Watch TV after breakfast and relax. If your family see you watching television, they would be less inclined to force you for shower. Grab some snacks after breakfast and eat them while you are watching TV. If you eat a lot of snacks, you will feel less hungry for lunch, helping you avoid washing up.
Do things which take time
In order to avoid washing up on a Saturday morning, the best thing is to start doing things which take time. You can start to clean your room and organise things. It will be satisfying for your family to see you cleaning your stuff and they would not force you for a bath.
Once you get tired, you can start to study in your room, which is another great way of avoiding a shower. If this does not work, you can go out with your friends and spend some time out; hence, eliminating all the chances of having to wash up.