Excessive force is the use of more force than required to assure the officer's safety and compel the requisite conduct by the subject of the force.
Most police departments have enacted as policy and rule some broad categories of appropriate levels of force, such as delineation of circumstances when an officer may use a choke-hold or draw his service revolver. Hence, a term you have probably heard: the "in policy" shooting or take-down. But, ultimately, excessive force will always be a fact-based assessment of the specific circumstances of the individual police officer-subject encounter, as enhanced by the 20-20 vision of hindsight.
It is important to note that information known to the officer but not to the subject may justify more force than would otherwise seem acceptable. Example: information relayed at roll-call describing a suspect who is believed armed and wanted for an assault on a police officer. If you match that suspect's description, and if you are dressed in a way that would enable concealment of the weapon, the officer may be allowed to use more force in detaining you than he could use absent those factors.
As for your situation when you were in high school, it is too late to reconstruct what information the officer had that may have justified extra caution and pre-emptive conduct on his part. And, as soldiers find in combat zones, the size and age of the suspect are irrelevant when firearms are concerned. So, it is not clear-cut that you were the victim of excessive force. But there are strong indications that you received an "attitude adjustment" in the field from the officer, and if you had consulted an attorney at the time, this incident would have been worth investigating as a potential civil action and/or report of misconduct.
It sounds like this happened a long time ago, when you were in high school. In this case, the statute of limitations has probably run for any lawsuit you could bring. If you want to be sure, consult an attorney that handles civil rights cases.
In any situation involving the police, the question is going to be: Was the amount of force used reasonable for the situation? They will take into account the totality of the circumstances. That includes everything - the type of crime they're investigating, the relative size of the suspect, the suspect's actions, demeanor, etc.
In what you wrote, as detailed as it was, there may be subtle things missing that change the situation. I'm not taking the officers' side or yours, just a discussion.
Although it wasn't exactly the crime of the century, they have an obligation to investigate criminal acts. Was the search of your backpack legal? Search issues are very, very fact-specific, so one small change in the scenario can change the outcome. While you were detained, reaching for the bag for whatever reason could have been seen as an aggressive move by you. How do they know what you're reaching for - a weapon or only physically preventing them from searching for whatever reason?
Did the officers act appropriately? You don't think so. Was their force excessive? That would depend on all the facts. Were they wrong to prevent you from having your inhaler? Again - it depends on the facts, timing, etc.
I know you're looking for an answer to the question, but it's not that black and white. Every case and every situation is unique.