The area used as the Twin Peaks High School field is just down the road from the filming locations of Glastonberry Grove, Windom Earle's cabin, Donna and James' picnic, Audrey and Jack's picnic and the scene of Cooper, Truman, Hawk and Albert walking down the road after Maddy's body is found. Basically, it's just an open field next to the road with some picnic benches. This is another example of brilliantly economic location scouting. Because all these locations are so close together, it's likely that they filmed this scene on the same day as they filmed the after Maddy death walk, James telling Donna goodbye and Major Briggs' and Cooper's campout. Dammit, I never found that location! It's around there somewhere... Below: two aerial shots of the field from different angles.