KB Homes has model homes all out here in California (formerly called Kaufman and Broad). While the homes are models, the model homes have no screens on the windows. I don't know whether this is because the screens get dirty very easily in California and are hard to clean or what, but if you go to a new development in California and walk through the model homes, the windows are locked and there are no screens (and no interior doors).
Also I recall that I read somewhere that the kids were out in the front yard installing sod to reinstall some sod after some incident with some sort of complaint about yard upkeep. This could have been the HOA or someone else, but cutting sod requires a knife and perhaps the kids kept the knife.
Also, the last thing they do before the developers turn over the models to be sold is install the screens and the interior doors. This model would have been the last to sell because they had to tear out the sales office that used to be in the garage and return the space to a real garage.
I am not sure what type of screens were installed, but in my own struggle with screens and cleaning them every year, some are really hard to pop out without some sort of screwdriver. The window frames are made with some sort of non-wood material so nailing would be hard to do, but they could have been secured close with some other type of contraption. I doubt the monster parents allowed the windows to be opened because the stench would have noticeable to those passing by, particularly the front window that the 17 year old escaped from.