The point of the matter is that you should act in a way that is becoming of a prefect, not do something completely outrageous as you have done with that journal post. You mentioned things in your journal that's considered to be 'too much information,' of which I think you know what I am referring to.

That, coupled with the fact that it was public enough for even the first years to read, really makes it look bad for all the prefects.

It is not to say that you are not allowed to express yourself, but it is more of a fact that you should have done so privately with Mr. Larkin.