I live a few doors down from Scholls Heights Elementary and there are tons of pleasant routes that can get you to McMinnville from the south-metro Pdx area. Personally, I have found the Reach-the-Beach route one of the most pleasurable. That route gets you to Amity, then its just a short jaunt down to McMinnville on back roads or 18.
I think that the closest tri-met access to Scholls Heights is the intersection of Murray and Scholls Ferry. Take max to the Beaverton Transit Center, then transfer to bus. Scholls Heights is approximately 1/4 mile down scholls from that intersection. You'll pass a shopping center w/ drug store & grocery, coffee and juice shops if you've forgotten anything or need some fortification for the ride.