Having lump in throat or throat tightness sensation (not in the heart) sometimes in p.E. At school. What might be causing it?

Globus sensation. You described "globus"--a situational lump or ball sensation in the throat. Correlation with p.E. Classes raises the probability that stress is the key here. Other entities to consider: cricopharyngeal achalasia (a tight upper esophageal sphincter), oro-pharyngeal dysphagia due to dysmotility, atypical gerd, local ulceration, extrinsic impression on the esophagus, ENT pathology, c-spine arthritis.