Any ideas on why dog is peeing in house after obedience class?

At nine months old, he has hit his "teenager stage". When he gets home, I agree he is marking territory. You may wish to make sure he "empties out" before he can come in and then, with love and praise - unless he takes that as an invitation to act out (I had one of those), put on his bellyband and have him go to his quiet spot/crate/bed/kennel. Is he intact? A friend of mine with a Great Dane said that (about the same age as your guy) their dog has really settled down after the operation.

If you want him to use the doggy door, maybe go back to "training" him to use it. That way he knows you still expect him to use it. Be certain that he fits through the door and it does not hurt or bother him when he uses it. Plenty of ear room, etc.? Some dogs do not like their ears to touch the door at all. Belly bands are wonderful things! Thick Sanitary napkins work well in them.

Oh, he may have another bump or two - developmental stages, new activities, etc.

Get some dog treats, every time the dog pees or poops outside he gets a treat with a ton of positive reinforcement. Also, when he shakes or performs other tricks he gets a treat with again positive reinforcement. If he pees or poops in the house say "NO" and take the dog outside (even if he is already done) to let him know this is where you need to do your business. He will associate peeing, pooping and performing with getting a delicious treat. Dogs learn quick, get him in the routine and he should be sorted in a week or two. Good luck!

He may be marking territory. Is this a large amount of urine, like normal, or is it just small amounts? If he is marking, getting him neutered may help (it will take several weeks for that to take full effect). If it is marking, you need to treat it the same way you did the original housetraining - make sure you are watching him so you can tell him No when he starts to do it.

your dog may be going inside because you keep the dog in the house locked?

otherwise your dog may have done its buisness in your house before it went to obedience class

and continues to go in the exact same spot as it will recgonise the spot by smell.