Hi Dian:
As Bob said, you CAN set up those two settings in M mode, but as Kevin also said you should set the ISO to auto to give the camera some flexibility. That said, by forcing yourself into both a fixed shutter speed and aperture you are making it harder for the camera to adjust to short-term changes in lighting conditions for you. I would strongly suggest switching to Av mode, keeping the ISO on Auto and let the camera figure out the shutter speed.
As regards aperture, I will further add the following. You should seriously reconsider the necessity to shoot at f/11. Given you are shooting with a crop-sensor camera, you should get quite a deep DoF at f/8 or even f/5.6, and by opening up each stop you have twice as much light to play with, and thus be able to achieve a higher shutter speed and/or better ISO. BTW, setting the ISO at auto will mean that camera will try to always use the lowest ISO possible for the other settings, so it is to your advantage to allow as much light into the sensor as possible.
The following image was taken of a gathering of 85 academics for a symposium at my local university. Would you be likely to have as many students in one class photo? All of the subjects are acceptably sharp, and I can assure you that organizing academics is like herding cats.
I took the image in shaded available light on a partly cloudy day, with the camera set in Av mode and auto ISO. They had only about 10 minutes for the whole shoot and there was the complication of a second photographer to get his shots. 
As you can see my FF camera was shooting with an f/stop of 7.1, and because you are using a crop-sensor camera you should achieve a similar DoF with a wider aperture, say f/5.6. That said, I would suggest doing an experiment to gauge how deep you actually need your DoF if your group is smaller, and then use a DoF calculator to confirm the settings you require. There is one here:
A Flexible Depth of Field Calculator (cambridgeincolour.com)
For example, if you dial the settings in the picture below, you will see that with the modest aperture of f/5.6 on your crop-sensor camera, at 24 ft from the nearest subject, you will have everything within an acceptable DoF from about 10ft to infinity.
If you have the ability to control the location, I recommend setting up with your students on steps or a stand, preferably in a shaded area, to avoid high contrast, wide variations in direct light, and having your subjects squinting against light - you certainly don't want direct back lighting. That gives you the added benefit of a reduced level of variation compared to direct sunlight, so things are more predictable.
