Keep him out. You are right to insist on safe lab behaviour. Call his parents or get them in for a meeting. Explain what is happening & say that you are worried (if you are) that he may be missing out on key syllabus content but that with his current behaviour he can't be allowed lab time. You have a 'duty of care' to the other pupils in the class & their safety should not be jeopardised by one person's silly behaviour. Have a word with him before you do this & explain to him exactly the same things as you will tell the parent(s). He may decide to change his behaviour before the parents get called in - don't use it as a threat just say that they need to know what is happening and it is normal to keep parents informed of pupils progress - in this case no progress - in the subject. If & when he does return, keep him under your nose & give him simple tasks that are easy not to mess up. If he does 'mess up' then it is back to exclusion or observation of others or whatever you decide. The other pupils will appreciate your perseverence, you will engender a productive lab environment & you will get a good reputation for effective classroom management which will serve you well for years to come. Explain to the HOD / your mentor or whoever needs to know what you are doing & why. Get them on board so the parents can't start playing one of you off against the other.