What will infantry school be like as a prior service SGT from the marines that already has a combat deployment under their belt goin army ?

Cadre will be aware or your background and expect you to prove yourself. The ego comment is correct, once they know you, expect to be held as an example for others in your unit.

If you are an NCO, you eill be treated as an NCO. You will still have to do all the training and meet the standard, but while you are a trainee, you are also an NCO. Keep that part in mind because you will also be expected to act like one around the new recruits to a certain extent.
Dont have a chip on your shoulder going in and there shouldnet be any issues.

source: Good friend of mine just went back through basic as an NCO.

But then again - you might have a drill sgt that has no deployment.

Looking at your mos I think you would be a great asset to the DS

Damn Devil, sold out....

You have to go because the Marine Corps and the Army do things differently just hang in there and get though it Semper Fi.

A young man I know well had a similar predicament. He entered the national guard at 17, still in high school. When he departed for initial boot camp over the summer, his grandfather, advised him to not discuss his family's military history. It worked well till the final week.
Grandfather decided he was well enough to attend graduation. grandfather was a veteran of the European campaign until the Bulge. He contacted Fort Campbell I believe, to arrange to billet and he was informed there was nothing available. As he was thanking them for their trouble, he was the consummate gentleman, the clerk asked what rank he was? Major General was the answer.
The conversation changed at that moment and suddenly there was any number of rooms to accommodate his needs.
So back to grandson. At the next morning's assembly the Drill Instructor announced to the entire platoon "I thought I told you to tell me if any of you had distinguished family?" The young grandson was shaking in his boots when the DI got in his face and read the riot act for not letting them know. Grandson's only defense was that the General told him not to. Damned if you do damn if you don't..

Well said and I’d say well informed.

Just learn to realize that you are an Ex-Marine and nobody special.

Determine to learn what is new or different and avoid comparisons "when I was in the Marines we did it to this way". You can be a tremendous force to help other recruits to learn procedures, accept hardship, and focus on the mission instead of their misery. In effect your example should be of stability. Your experience may best be served in the basic training environment not by being a know-it-all who looks good and makes others look less good by comparison, but by helping everyone in your squad to be better as a team.