Inaccurate. Mary doesn't say anything about what would happen if schools tried to enforce this requirement. She just says why the requirement would be impossible to enforce: there aren't enough jobs to begin with. To infer from this that there would necessarily be undesirable consequences if schools tried to enforce the requirement anyway would be to make assumptions going beyond the stimulus. Maybe the requirement would just exist on paper and be impossible to follow, without creating any undesirable consequences for students or the job market.

While Tom does seem to think the trend of high school graduates lacking vocational skills is avoidable — after all, he suggests a solution to the problem — Mary's response isn't about whether the trend is avoidable or unavoidable. She just points out a problem with Tom's proposed solution.

Mary doesn't disagree with any of Tom's stated premises: the fact that employers are complaining, or that vocational skills are best acquired on the job. Her response targets an unstated assumption behind Tom's main point.

This is correct. Tom claims that requiring high school students to work part-time jobs will help solve the problem of high school graduates lacking vocational skills. This assumes that jobs are available for these students. Mary undercuts this assumption by pointing out that there aren't even enough jobs available for the students who already want to work, let alone for all the students who will be required to work in Tom's proposed scenario. So instituting this requirement won't actually help solve the problem.

Mary does not propose an alternative solution to the problem. Her argument is limited to addressing Tom’s proposed solution.