We've always locked up medications for basic safety reasons, but we've friends who've learned the hard way that this is a good idea.
There are two reasons to keep meds locked up. One is accidental or misguided misuse by a young or judgment impaired child. I've read stories of high functioning ASD adolescents overdosing on sedating medications to help them sleep.
The other reason, of course, is that many behavioral meds have street value and are widely abused. They can be a powerful temptation for teenagers.
So lock 'em up.