The victim was driving behind a small black car in Theale Road at about 9.20am on Monday when the car in front suddenly slammed on its brakes outside St Mary's School.
The female driver got out and shouted at the victim through the window, grabbing at her arm, before the victim managed to wind her window up and the woman drove off.
PC Robert Simpson-Jones said: "This incident happened just after school dropping off time and I am hopeful some parents might have seen the incident or recognise the description of the offender or her car."
The woman is white, about 45-years-old and medium height, with short blonde hair.
Call 101 or Crimestoppers anonymously on 0800 555 111.