Yes, public courts are often reserved for lessons, drills, etc. There should be a sign posted on what time and for what purpose the courts are reserved. Sometimes, though, the parks department is too unorganized to actually post the appropriate sign. The guy may very well have been teaching a parks department class for which he got paid little, if anything, because he wants to help kids learn tennis. Maybe he is just doing this on his own for free to help the neighborhood kids.
Of course you are far too important to take the needs of some lowly ragamuffins into account.