That means you don't have to start from scratch with your teacher.

from 是介词，表示"从...开始"或"来自..."，在这里表示起点。 scratch 本义是"抓、挠"，引申为"起点、零基础"，因为抓挠是最基础的动作。 from scratch 是固定搭配，意思是"从零开始""从头做起"，指不借助任何现有基础重新开始。