We are using Forms in school to create assessments for students. It works great if students finish in in one session. We have run into problems with students who need to finish the next day. If they "submit" it unfinished, their incomplete questions are marked wrong. If they just close, their progress is not saved.
You could possibly divide into multiple forms or make the tests shorter, I know that doesn't really take into account if they skip over questions - something I myself do in exams. However, there are ultimately no real effective workarounds - Microsoft need to allow the Form to be saved with the answers and then continue on a different session before it is submitted. You may want to consider a third party solution for a temporary time such as SurveyMonkey which allows you to do this