Under 1.410(b)-6(f)(1), there are five conditions that must be met in order to treat an employee as excludable:
i. The employee does not benefit for the plan year,
ii. The employee is eligible to participate in the plan,
iii. The plan has an hours of service or last day requirement in order to accrue a benefit,
iv. The employee fails to accrue a benefit solely because of the failure to meet the hours of service or last day requirement, and
v. The employee terminates employment during the plan year with no more than 500 hours of service.
"Plan" for 410(b) purposes means after the application of the mandatory disaggregation rules, so not the entire plan, but just the matching or profit sharing portion of the plan. See 1.410(b)-7(c).
Leaving condition (iv) aside, the counselors in your example satisfy condition (i) and (v) but not condition (ii), and condition (iii) is also not satisfied. Therefore the counselors cannot be treated as excludable for purposes of the coverage test.
However, the plan will likely satisfy the coverage test by disaggregating the portion of the plan covering otherwise excludable employees, that is, the employees who have not satisfied the maximum age and service conditions under 410(a)(1). That will include the counselors and presumably some of the FTEs, however as long as that group does not contain any HCEs it will automatically satisfy the coverage test.
For that matter, you haven't said anything regarding the relative number of HCEs and NHCEs in the employee population. You said it is a school, so presumably there are not a lot of HCEs. Would the plan pass the ratio percentage test, even without disaggregation? How about the average benefits test?