Letting your son have alone time on the computer is not a good idea. Pornography sites target kids, they set the name of the site to be slightly misspelled from the real children's site and boom the kid is exposed! Never ever let him on there alone.
That said, even if the games are not online and he has no access to the internet, that much screen time is not good for his developing brain, even if its "educational". He needs to be moving and playing and learning how to use his body! His teacher is spot on, don't make her job harder!!!
10-15 min a day is plenty! You might need to get a more interactive toy for dad to have him play with when you're shopping, or maybe you should take him to the store with you. I do all my shopping with my 3.5 year old and 13 month old in tow. Yes it takes a bit longer, but they're learning how to behave when I'm shopping. Basically if dad is letting him on the computer for hours while you're gone, this is a problem, and if he doesn't see it you need to take your son with you because the temptation to put him in front of the computer so he can do what he wants is too great. What's telling is that you've had to lock the door to the computer room. Obviously your son is SO interested in the computer that he's willing to sneak in and use it against the rules.
Best wishes!