Pentax AF280T (always set to green two-level auto flash operation, head tilted 40 degrees up)

If you don't have one, you can download a .pdf Pentax AF 280T Manual.

Forty degrees is not high enough for bounce flash in most situations. The light from your flash appears to have bounced off the ceiling and come down well past your intended subjects. Either that or the flash visible in the scene is the direct light from the lower portion of the flash's output. You lit the rear walls up pretty good in some of them!

For the closer shots, bounce flash needs to be pointed almost straight up, between about 75 and 90 degrees. For the larger groups that are a little further from the camera, somewhere around 60-75 degrees should work.

If you want to give a little direct light in addition to the bounced light, you can use a "bounce card" or even a small diffuser attached to the flash with an elastic band or velcro strap. A 3 x 5 or 4 x 6 index card and a rubber band or two-sided tape make a good "bounce card" in a pinch.

For quick and dirty, run and gun, on-camera flash for events, I really like this inexpensive two piece set I bought a while back. I use the smaller one on-camera more often than the larger one. I do use the larger one for off-camera flashes. It has flexible metal rods that allow the reflective surface to be shaped. It can also be rolled up and used as a snoot.

P.S. I'm sure you want to learn how to shoot film, and I wouldn't dream of trying to talk you out of that. But for learning purposes regarding flash, if you have access to a digital camera that can trigger your flash you can accelerate the learning curve and spend a lot less learning how to do flash with the instant feedback and lower cost-per-click of digital. Just use manual exposure mode, set the camera's ISO to the speed of whatever type of film you typically use, set the shutter speed to 1/60 (since that is your film camera's sync speed), and set the lens to apertures you would typically use with your film camera.