Toddler Physical Agression

My 20 month old boy has been struggling with scratching, pulling hair and hitting when he is in a large group of children like the gym child care. He has been kicked out of the nursery a couple of times because of this behavior toward other children. At home, he does display similar behaviors but we intervene and give him a stern “no” answer and explain how that behavior will not be tolerated. Is this just a phase or should I/my husband be doing something else to help him understand his cause and effect behavior? He’s going to start a 2 day/week preschool in September and I’m concerned that these behaviors will be an issue when he starts there.