In the evening so the lighting to me is difficult to work with.
But that is the best hour!
I feel you are taking your photos by "chance" if the light is good you take a good image.
The step you need is to control the light. And that can only be done with artificial light. Get a speedlight.
Take a glimps at a simple image search: https://www.google.com/search?q=flash+and+ambient+light
1) First, you need to practice a one light portrait full body with a speedlight and a diffuser. I strongly recommend one softbox instead of an umbrella because an umbrella catches the wind like a parachute.
Know the angle, start with a 45-degree angle and build from that. You can practice in an indoor environment.
2) Then you have to know how to combine the speedlight with outdoor environment light. There are several tutorials online, but the basic idea is that you combine:
- Iso Speed + Flash power + Shutter speed
to achieve good results. I leave aperture out of the equation because that is what builds the style.
3) Depending on the 3 previous variables you probably need a 4th one, either a Neutral density filter or a High-Speed Flash.
And one thing that you need to control is color temperature of this flash, so some orange warm gels will be handy.
So My list of recommended gear:
At least 1 speedlight (and a dozen of rechargeable batteries, 2400 mA)
1 Sturdy tripod with a sandbag (which you can make yourself)
1 softbox or octobox (I would recomend a big one, like 60x90cm)
Some pices of color gels (orange, to simulate golden hour)
An external lightmeter would be great.
You can not work against the clock! You only have like 30-45 minutes to do that, wich is impossible.
You need to take a good picture regardless of the time hour. Some students will have a great evening photos, let others have a great night shoots and other great daylight shoots!
The basic idea to combine flash with environment light.
A) Define the look of your photos, choosing for example, an aperture to have bokeh or not.
B) Choose an appropriate flash power depending of the distance to your subject.
C) Play with the shutter speed and iso speed to have an interesting exposition of the environment illuminated with the ambient light. This will be changing all the time as the sun goes down.
4) Have more room to have a wide aperture using the ND filter or the HSS, specially when you still have bright daylight.
An additional thing I should say, is that you need to work on your composition.
On the second image, you can really benefit from the orange gels to make this light warmer.
And the additional recommendation that you shoot in Raw. Oh... and a walking tripod is handy too... commonly named assistant.