The status of Mosaic Law during Jesus' lifetime

How can the following verses be reconciled? Matthew 5:18-20: >“Amen, I say to you, until heaven and earth pass away, not the smallest letter or the smallest part of a letter will pass from the law, until all things have taken place. Therefore, whoever breaks one of the least of these commandments and teaches others to do so will be called least in the kingdom of heaven. But whoever obeys and teaches these commandments will be called greatest in the kingdom of heaven. I tell you, unless your righteousness surpasses that of the scribes and Pharisees, you will not enter into the kingdom of heaven.” Matthew 23:1-3: >”Then Jesus spoke to the crowds and to his disciples, saying, “The scribes and the Pharisees have taken their seat on the chair of Moses. Therefore, do and observe all things whatsoever they tell you, but do not follow their example. For they preach but they do not practice” Matthew 12:1-4: >”At that time Jesus was going through a field of grain on the sabbath. His disciples were hungry and began to pick the heads of grain and eat them. When the Pharisees saw this, they said to him, “See, your disciples are doing what is unlawful to do on the sabbath.” He said to them, “Have you not read what David did when he and his companions were hungry, how he went into the house of God and ate the bread of offering, which neither he nor his companions but only the priests could lawfully eat?”