It is difficult to change one's behaviors. Neural pathways are dedicated to maintaining repetitive actions and behaviors. The opposite is also true. Enacting positive behaviors are beneficial; they are hard to stop doing. It is important to enact firm expectations to foster desired behavior. Dramatic changes in behavior rarely occur outside the case of epiphanies or significant life changes. For the average individual, slow and steady change, with clear expectations, is much more reasonable.
To understand why change is so difficult, let's explore psychological reactance. When someone is told to change their behavior, they may react negatively. Regulations and rules threaten an individual's freedom; teenagers are at the point in their life where they most value this. Initially this is mostly an emotional reaction. Consequently, teenagers fall prey to reverse psychology quite easily, unaware of what they agreed to.
Setting standard expectations results in social norms. Norms become informal rules that are simply understood by the individuals that follow them. It becomes an automatic process that does not require emotional or logical input to reinforce. Norms don't suffer from the effects of reactance since norms are expected and do not involve attacks on individual freedom. The only difficulty comes from the initial establishment of these norms.
Adolescents (and adults) react negatively to infringements on freedom.
Be careful of restricting an activity ("don't do that"). It may reinforce the behavior or opinions contrary to what was desired. It may also further increase resistance to persuasion.
Behavioral norms are often automatic and don't need to be enforced.
Establish expectations early on and reinforce them early. Modifying behavior is very difficult. Set high expectations (that can be reduced) instead of moderate expectations (that need to be upped later).