In-class writing for English or history has its stresses and fear of running out of time is top of the list. Help her out by talking about how to pace herself so she isn’t scrambling to finish or having to stop two thirds of the way through.
The typical high school period can be from 45 to 55 minutes. Tests can range in format from one fat essay for the whole test to 25 multiple choice questions, two short answers and one short-ish essay.
Whatever the test format, she should break down the time period based on what she thinks she needs for each section. If she guesses she needs about 25 minutes to do the multiple choice, she can then plan to use the remaining 20 or so minutes to write the essay (perhaps 1 short intro and 2 body paragraphs). And then she can think about how to use that 20 minutes to write the essay. Say, 5 minutes to plan, 3 minutes for the intro and the rest of the time to write the essay (using the last minute to scan for errors if she has it).
Or it could be a different plan – perhaps leaving a bit more time for planning the essay or only setting a marker for when she needs to move on from the multiple choice section. Your approach with her can be to create whatever plan eases the stress the most and gets the test done in a way she can live with.
Plan to tweak the pacing as the year goes and she takes more tests. After each test, consult to see how it went and what needs to be adjusted. Pacing does mean being comfortable watching the clock – a skill that needs practice for some students. Practicing at home, if she is game, would help.