If this is speech, not in writing, I would only be concerned about how the beginning sounds like you are telling or informing your boss of what you are doing, and not asking.
My initial assumption is that the boss has some measure of control and the proper social thing to do is to ask if you may have permission to take off early, due to the listed reasons, even if you pretty much know they will say "yes." If that is the case, I would re-word, especially the start, into something more like:
I would like to leave work today early for my son's parent-teacher conference, if that is OK. I will make sure my work is done before I leave.
or, as a question
Would it be a problem if I left work early today for my son's parent-teacher conference? I will make sure my work is done before I leave.
NOTE: The changes I made to the second sentence are not any more correct. Either works. My suggestion only adds a little to the impression of more personal control over the work.
However, it is also possible that your workplace allows its workers to be more independent (as mine does) and perhaps you only really need to inform them that you will be out early. In that case, I would add very little, while still giving the boss an option to give a good reason to stop you:
I am planning on leaving work today early for my son's parent-teacher conference. My work will be done before that.
And, finally, from my beginning here I stated, "If this is speech ..." because -- as you have probably already noticed -- in writing, I would make these completely separate sentences to represent separate thoughts: addressing the timing and, separately, the work. Technically, you could also separate them with a semi-colon (;) if you really wanted to, but a period would do the job in a more common and simple manner.