I find it wrong all together, but yes, you are so right. If he didn't consent to such, then how can they do that?
In the case of a sudden death, they don't need consent to do an autopsy. Part of the autopsy procedure is to remove the brain and store it for later examination. It wasn't there for display, it was just being stored and the teens touring the morgue saw it.
They probably saw a lot of things, they may have seen an entire body. I think the morgue/coroner's lab was just doing what they always do, following their procedures. Maybe kids shouldn't be having a field trip there. I'd bet the kids or parents had to sign something saying they could handle whatever they saw.