Your question is one that is asked often by parents of young people taking the SAT or ACT college entrance exams.  Statitically, test scores do not change that dramatically from one testing cycle to another.  There are exceptions but generally major change in scoring is highly unlikely.

Some states/local school systems test students after these students have been given intensive assistance.  This might make a difference in improved achievement scores.

You will need to find the right balance between practicie assessments and how helpful they are in your individual situation.  Good luck.