Classroom observations can be difficult, but at least a good quality form will make it easier. When taking ABC data, the idea is that we are trying to figure out the function of the behaviors. Typically the best ABC data is taken when we are in the moment, but sometimes we can take this data after the fact. For the best results, record the data as you see it. I typically watch and write down the behavior first, then depending on the consequence's speed I will jot down that or perhaps quickly note the antecedent (trigger) first. Keep in mind that behaviors can chain. Some consequences (or the absence of those consequences when they are withdrawn such as teacher or peer attention changing to another student) can be the antecedent to the next behavior.
1) Don't just record behaviors of note (a.k.a., undesirable behaviors). If the student is working or not engaging in the behaviors of note, what are the ABCs for those as well?
2) When taking observation data it is best to be unnoticed by the student, or if you are noticed take data a couple times before or be present in the room frequently with a data sheet close by so she/he is used to you being there. This will help in preventing the student from changing their behavior in your presence.
3) Make sure that when you are taking behavior data that you are defining what the behaviors are clearly, unemotionally, and functionally. Saying a student is being disrespectful is emotional and does not define the behavior well. Saying the student is smirking, walking away when instruction is being given, is arguing/talking back, shouting out, are all examples of being functional in defining the behaviors. Functionally defining the behaviors will make it easier to complete functional behavior assessments (FuBAs) and write behavior intervention plans (BIPs) for the student's IEP. What is more, I usually have an my Brief Time Sampling Behavior Sample Sheet (Classroom Observation Form) with me when I am doing an observation so that I can get a head start on collecting that data as well.
Note: This sort of data collection is research-based. A link to the research is at the bottom of the form if you wish to learn more.