That can take a little patience, as even if you nail it indoors it is very easy for a dog to get distracted when he's out and about. For the first 18 months or so I had to clock my dogs attention before he noticed something he could run too! As long as you have patience and keep reinforming their behaviour (cheese and peanut butter work the best on my idiot) then you'll smash it!
In fact I forget you're a teacher, this should be a piece of piss!
Cheers for the advice... You do develop patience as a teacher, so I guess I can use that! But, yeah, keep at it, seems to be the advice.