Hi, no one can say how long it will take for the first wheat etc. but you are off to the right start by searching property that is old enough, the 50's school has probably been slammed as everyone knows where it is. That's not to say you can't find anything there but your odds are greatly lessened both by it's lack of relative age and high detecting traffic. One thing that may help in general is to try to look in areas on your site that look less inviting. What I mean is areas that aren't as manicured and easy to hunt. My first time out at an old school that was built in 1909, I found a walking liberty quarter, 3 buff nickels in one hole, and 4 or 5 mercs. Ideas hunting in an area that the school had stopped really maintaining- it was heavily grassed, weedy and trashy too. The nice part of the school grounds had been pounded but because I was a newbie and worried about my digging technique- I hunted the part no one else wanted to and was well rewarded. The Griffing technique someone mentioned earlier can be invaluable. When I first started doing this in 1977, we dug every signal pretty much. Some detectors had discrimination but it affected depth so much. I used to grid a trashy area off and get all the obvious signals dug, then listen for the whisper of something deep and sweet. It worked too. Now I am old and like everyone else these days impatient. I have no doubt that I miss some good targets through the use of 2 much discrimination. A case in point is a buddy of mine who took a guy he worked with out hunting for the first time-using my friend's back up detector. I called him up afterward to see how they'd done. My buddy said the guy was an idiot. So I said ya'll didn't find anything then. He said, I didn't but that idiot found a trime and a half dime- neither of which I'd ever heard of at the time. So I said doesn't sound like an idiot to me and my buddy said "yes he is- he wasn't using enough discrimination" I said what? He said, I couldn't get a pick up on either one in the ground- if he'd been using enough discrimination he would have knocked it out too. That taught me a big lesson that I'd once known but forgotten. If the sight is old enough it's worth it to cut back on discrimination- a lot. Sometimes, I don't use any except iron mask, usually run it just higher than foil. Do I dig everything? No, I trust my ear and experience with my machine to pass over some things but I don't want to miss any trimester or half dimes either. Oh, by the way, I went back to same area of park they were hunting and pulled out a man's gold nugget ring. Don't know if my buddy swung over it or not but if he did- he would have knocked it out as a pull tab.
Just give it time and keep swinging. 
