I attended Notre Dame High School in West Haven beginning in the Fall of 1981. Fantastic school. Ultimately, I would only go there for both my Freshman and Sophomore year, and leaving there is actually a big regret of mine. Frankly, if memory serves, there's just no way any student - especially one suspended, never mind expelled - was able to simply strut into the building midday. Pre-surveillance cameras aplenty, doors would be locked, with a set of Brotherly Eyes on the front one at all times.
In lieu of "actual security guards and/or school resource officers," there was (in no particular order):
Dean Nagle
Brother Benjamin
Brother Joe
After transferring to East Haven High School for my Junior year, in the Fall of '83, things would get dicier, but there was still security to tend with. The sharp contrast between a private school and a public school was not lost on me, in large part due to the student to my left now suddenly being a Lisa or Cindy, sporting skintight Jordache jeans and smacking gum. The fights were more frequent and lasted a bit longer, and didn't result in a Brother swatting you afterwards with a ruler for a few minutes, but aside from that it was just as monitored a building as ND. (With, admittedly, coming and going perhaps the occasional occurrence.)
In lieu of "actual security guards and/or school resource officers," there was (in no particular order):
Stormin' Norman
A guy we just called "Gomez," who would literally chase you
Three - count 'em 3 - old ladies known as "hall monitors" (or some such), there on a volunteer basis, who patrolled like nobody's business, and would see a kid who had been expelled pull his vehicle down the side street and get the word out before he ever got into the actual parking lot - let alone the building.
But in Florida this expelled 19-year-old got into the building armed to the nines and up a few flights of stairs, plus pulled a fire alarm???