You could check out the English Language Learner Motivation Scale (ELLMS) by Ardasheva et al. (2012). The scale was developed and validated specifically for measuring ELLM among pre-college students. Here is the full citation.
Ardasheva, Y., Tong, S. S., & Tretter, T. R. (2012). Validating the English language learner motivation scale (ELLMS): Pre-college to measure language learning motivational orientations among young ELLS. Learning and Individual Differences, 22(4), 473–483. https://doi.org/10.1016/j.lindif.2012.03.001
: Robert Gardner is certainly a well-known source for quantitative tests of L2 motivation (and L2 anxiety). You could simply use the motivation scale from his Attitude Motivation Test Battery/AMTB (if you are looking for "a standard one").