The system is listed as LTR but in the mode column is says NXDN. Confusing...
Harlandale Independent School District
Harlandale Independent School District
LTR Standard HISD operates on this trunked system.
Frequency License Type Tone Alpha Tag Description Mode Tag
460.05000 WQKF288 RM 16 RAN HISD PD-1 Harlandale ISD Police Dispatch
The talkgroup on the LTR system, for the HISD PD, is noted as
PD Dispatch (NXDN Backup)
I take that to mean that, primarily, they use the NXDN conventional channel, but still can access the specific TGID on the LTR system if, for whatever reason, the NXDN is not available.
But, as already noted, since schools in Texas are currently shut down, the fact that nothing is being heard does not really mean anything in particular. When & if schools reopen, after the Covid-19 danger is passed, or well under control, then at that time it would be more likely that the NXDN channel, and perhaps the LTR system (for other school functions), would be active again.
If the OP wants to try something else, to verify whether or not NXDN is working, then try this business NexEdge ayatem:
BearCom (Central Texas NexEdge) Trunking System Profile
www.radioreference.com