Hi stthomas,
Thanks for your question, and welcome to the forum! I think you will find it a very useful resource.
I think we can diagram this stimulus this way:
If something should be taught in secondary schools, then it is useful.
should be taught
useful
Abstract knowledge is rarely useful.
abstract
not useful
The conclusion is that we should replace teaching abstract knowledge with teaching students how to evaluate practical arguments based on science. However, we know from the first rule that anything that should be taught must be useful. Therefore, for this argument to be coherent we need to know (as E makes explicit) that evaluating practical arguments based on science is practically useful.
I hope that helps! You are correct that useful is a key word here: it is, in brief, the necessary condition that must be satisfied for us to reach our conclusion, and therefore is an assumption on which the argument depends.