Since I have been trying to get our school to do one for my son for the past 2 years with limited success I am a bit shocked at the hostility to the idea (and the "ugh, behaviorist usually works with autistic kids, can you believe that!" part is... a bit off-putting). Glad ElizabethN had a decent experience, at least.

The goal of an functional behavior analysis is to identify what triggers a behavior and then implement a plan to work on the triggers, rather than the behaviors themselves out of context. If done correctly the outcome for the OP's child should be "child finishes work early, child gets bored, child misbehaves" and the ideal behavior improvement plan would be to make sure the child is not bored (differentiation, acceleration, grade skip, GT pull-outs...).

Of course there are many ways an incompetent or hostile observer could do this incorrectly -- there is one person at my school I wouldn't trust with the process, and if the relationship with the school is truly broken having a private behaviorist spend the time needed to get the full picture at the school might be the answer. But working outside of school with a private therapist to fix the child when what is clearly needed is to change the environment just doesn't make sense.

Anyway. For marytheres, the IEP forms in our district always start a goal with the baseline, ie. what the present level is. AFAIK this is good practice, and exactly what the Wrights recommend when writing SMART (specific/measurable/action words/realistic and relevant) IEPs.

You might want to get more details on who will assess the baseline, what the methods used will be, and shape the process (ask for a parent questionnaire to be included, make sure that 1:1 assessments are done with somebody you/your child like, make sure observers are not biaised -- ie. that the horrible aide is not the one taking notes...), but refusing to get a baseline... just doesn't make sense to me.