Dear Natalie,
I teach college chemistry, but I am sure that the high school level will be similar in terms of the atomic weights. We use the "Conventional" weight, as that is the one defined by a single value from the weighted isotopic abundances with defined error and tolerances. The "Standard" weight as defined by IUPAC is actually a range in many cases, with more detailed description of the variations encountered in different samples. A full discussion can be obtained here: https://www.iupac.org/publications/pac/pdf/2011/pdf/8302x0359.pdf
In all cases, and particularly for secondary education, the values given are more precise than they need to be for most calculations. The Standard Weights would normally be used only in more specialized or precise research work. Other than an example of how the weights are determined, and an excellent description of the measurement and sample errors involved, they would not be used as values in early academic work.
A quick check of most available Periodic Tables online or in textbooks will show that a single value is given, which is the "Conventional" weight.
Best regards,
Steven
