I spent many years giving and evaluating standardized tests in my teaching career (I trained in Special Education), and I think most people greatly misunderstand them. We are not required to give tests in my province, but I think it would be important to know what the district does with the scores. What is considered 'low'? When do they 'intervene'? Does there have to be a pattern of low scores, or does one low score ring alarm bells? Does the district do anything with the scores at all?
Percentile scores are very different from percentages. Tristan mentioned that 25%ile is a failing score in essense. Perhaps the test Tristan is talking about is different, but in every test I've ever administered, 25%ile is actually considered average (although on the low end of the 'average' range). The percentile is a ranking out of one hundred. So, a score of 50%ile means that if 100 students took the test, 50 would score worse than you, and 49 would score better (you are the last one, to make 100). The 'middle' 50 students taking the test are numbers 25-74 (with 25 above them and 25 below). So if you score 25%ile to 74%ile, you are in the middle half of the group, and thus are considered average. Below 25%ile is below average, and sometimes the scores have ranges that are called 'below average', 'fair' and 'poor' or something similar (with similar labels for scores from 75%ile and up - above average, excellent, etc.) 25%ile doesn't mean you know 1/4 of the material, or that you got 1/4 of the quesions correct. It's all just how you rank with everyone else. There shouldn't be any talk of a problem unless the student scores significantly below the 25%ile. This is where I assume each district will make their own distinction about when they are 'concerned' enough to take any type of action with a certain child.
Again, each test is different, and I can't see yours, so I can't give you absolute information, but that just brings me back to my original statement: find out what will be done with the results. Unless a low score is going to jeopardize your ability to teach your child in the way you want to teach, then don't worry and continue on as you were!