The effects are both short term and long term. In the short term, there is a dropping off in concentration in the classroom - once the class realises that the teacher, for example, has no way of making them work harder, there is a general feeling that there's no need to make the effort. Then you get a lot of time wasted on small things - a classic example is the class where a lot of students turn up late because there isn't an effective lateness policy, so everything gets disrupted. In the same way, the teachers will spend a lot of time arguing with students about things that shouldn't need arguing over - forgotten homework, where to sit, etc - whereas in a well run classroom or school there is a policy on all these routine things and everyone knows what it is.
In the medium term, what typically happens with older children and adolescents is that the 'troublemakers' take over. Learning takes place only if the loudest and most disruptive elements decide to allow it - which of course means that it rarely takes place. The teacher's time and energy is spent dealing with the troublemakers, persuading them to behave or making threats that everyone knows will not be carried out. In this situation many teachers will think that a lesson has been successful if they just managed to get all the class to sit in their desks, be reasonably quiet and not start any fights, regardless of whether they learned anything! This is the same as if you turn up at work, stay at your desk reading all day and then go home thinking you have achieved something.
In the long term, standards go right down in an undisciplined school. The students who want to learn find that they are actually being prevented from doing so; they see that the only way to get the attention of the staff is by behaving badly, and there is no time or attention left over for encouraging hard work or creativity. You also get a huge increase in bullying and fighting in such schools, and quite often crime and drug related problems will also escalate in an environment where the school isn't able to offer any leadership or give any help to young people who may be going in the wrong direction.
Several other Blurters ahve written about this subject too, so it's a good idea to explore some of the other answers that come up on this site.
In the medium term, what typically happens with older children and adolescents is that the 'troublemakers' take over. Learning takes place only if the loudest and most disruptive elements decide to allow it - which of course means that it rarely takes place. The teacher's time and energy is spent dealing with the troublemakers, persuading them to behave or making threats that everyone knows will not be carried out. In this situation many teachers will think that a lesson has been successful if they just managed to get all the class to sit in their desks, be reasonably quiet and not start any fights, regardless of whether they learned anything! This is the same as if you turn up at work, stay at your desk reading all day and then go home thinking you have achieved something.
In the long term, standards go right down in an undisciplined school. The students who want to learn find that they are actually being prevented from doing so; they see that the only way to get the attention of the staff is by behaving badly, and there is no time or attention left over for encouraging hard work or creativity. You also get a huge increase in bullying and fighting in such schools, and quite often crime and drug related problems will also escalate in an environment where the school isn't able to offer any leadership or give any help to young people who may be going in the wrong direction.
Several other Blurters ahve written about this subject too, so it's a good idea to explore some of the other answers that come up on this site.