There has to be SOME conflict in Hogwarts because the trailer clearly shows Bellatrix walking along one of the house tables in the Great Hall! They probably didn't big up the fight at all though, like they normally would, so it didn't take anything away from the last film.
What surprises me is how the running time is 153 minutes. Order was a lot shorter film and suffered for it, so maybe David Yates learned something. It infuriated me how they cut so many emotional beats out of Order that should have been there. The Lost Prophecy part at the end was, up to that point anyway, the best chapter JK Rowling had ever done, and they cut 90% of it out. They didn't even include the Prophecy, you just hear a small part of it when the sphere smashes.
Interestingly, they list Richard Griffiths and Fiona Shaw (the Dursleys) as being in the cast for Half-Blood Prince. Thought they were cut completely.