In this verse, "schoolmaster" is a stylish interpretation of the word:
paidagógos: a trainer of boys, i.e. a tutorOriginal Word: παιδαγωγός, οῦ, ὁ
This "pedagogos" was a strict person, one who would harass the young people going to and fro from school to home, making sure that under no circumstances did they deviate or fall away from their proper path.
In a sense, the law, this "pedagogos" was a "bully" of sorts, a standard which no one could ever meet, an ever-demanding and unrelenting pushy piece of stone that would brings us to the end of ourselves.
We are not defenseless, though. The law may bring us to the end of ourselves, but then:
"For ye are all the children of God by faith in Christ Jesus." (Galatians 3: 25-26)
We were alienated, beaten down, but the Jesus Christ our Savior has come, and He has taken all of our hits for us, He took our beatings, He fulfilled the law, whose requirements were beyond our capacity to fulfill:
"And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;
"Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross;
"And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it." (Colossians 2: 13-15)
This same law was the weapon of Satan, which he wielded against us because God could not go back on His covenant with Adam, in which the moment that our first parents ate from the Tree of the Knowledge of Good and Evil, they would surely die. God does not impart sickness and death into our lives, but Satan had this power because Adam bowed to him and let him take Adam's place as the God of this World.
The same faith that makes us Sons of God is the same faith that protects us from the fiery darts of the Enemy:
Know that you are a child of God, the "bully" of the Law can never hurt you again! Just believe on Him!