An average typer should be maintaining an accuracy rate around 92%, about 8 mistakes per 100 words. To improve on accuracy, students should slow their typing to focus more on typing correctly rather than quickly. As the amount of mistakes lowers, only then should students be pushing for a higher WPM.
When measuring a typists accuracy was a relatively new idea, the rules in place were quite harsh! When typewriter typists were tested on their ability, their WPM score was reduced by 10 for every mistake made within a minute long test. So a speedy typist may clock in at 50 WPM but make 3 mistakes, dragging their overall score down to 20 WPM. Luckily the metrics in place these days are more nuanced in their scoring. The Typesy Mastery Metric, for example, scores based on speed, accuracy, and touch typing knowledge.
Regards,
Alex (The Reimagined Classroom Teacher)