Why didn't Sirius apparate?

[sylviablundell2001]

No: HPFGUIDX 82442

My 12-year-old has asked me a question I find difficult to answer.  
Perhaps someone could help.  The question was: why did Sirius go 
through all that trouble, swimming for miles and then journeying 
north when he could have simply apparated once he was outside the 
prison and re-appeared outside the grounds of Hogwarts.THEN he could 
have slipped in disguised as a dog.Would he require the wand he 
presumably didn't possess to apparate? It all sounds too simple, so I 
suppose there must be some good reason why he didn't use this method.
I'm probably overlooking something obvious, but would be glad if 
someone could tell me what that is.
Sylvia