Drill rate should never decrease but there should be max total BP.
Default structures can't be killed or deconstructed.
Player-built structures can be bled and are targeted by bots.
This is so players can learn to cooperate (you'll need to defend a forward) and build (at least basics like no exposed eggs in the middle of big rooms) and to shorten the escape routes so that training games can be finished faster.
I'm actually in favor of closing "official" training and having murderer take over completely.
I disagree with lamefun's first point though – ideally the training server should run gameplay as close to vanilla as possible so that new players learn the standard behaviour.