Aaahm, I am otherly convinced, that for expressions/portraits the timed class mode with the 30 sec and 1 mins shorties doesn't work well. I personally always change to the "All the same time" mode for that, select 3600 secs as time limit, and change images manually.
The most commonly cited basic head construction is from Mr. Andrew Loomis, first developed in "Fun with a pencil". You can still find the original pdf for free on the interwebs, and tons and tons of people expanding on it. Caution: Loomis invented a quick method to draw relatively impressive looking faces with little prior practice, but they are all "from imagination", in other words, a bit random.
If you want to learn how to match Loomis' heads with a specific reference, I advise to start with repeating his basic construction without reference a lot. That is exactly, why I think a quick gesture timed class makes no sense for it. You do your "shorties" basically offline, from "imagination", without reference. And drawing the Loomis construction properly will take you way more than a minute anyways, so if you tried a quick gesture mode, you would only ever end up with a few initial circles.
Also search for images of Loomis head, and you will find some highly stylized, robot head like looking derivations of Loomis method, that is what a lot of professional use for solving head placement, lighting etcetera.
After you repeated those offline practices often enough to feel confident with Mr Loomis construction, and want a reality check, try applying the method to drawing from reference. When you are confronted with a real person's face, with all its specifics and flaws, you will start the process to learn how to match Loomis idealized rhythms to reality. And if you try to do it on a timer, you will just revert to what you practised so far, and produce idealized, non-distinct features, which you could have done offline as well.
BUT: Setting the timer to infinity doesn't mean that you should ignore the basic benefits of quick sketching, you just have to do it yourself. For example, if you spent 5 to 7 minutes to do a head construction, and then 50 more minutes on embellishing it with a lot of details, to only then realize, that it just looks crappy and always will, then you wasted basically an entire hour to effectively practice head construction for 5 to 7 minutes, and 50 minutes practising to polish a turd, which is utterly useless. If you had stopped yourself in time, you could have done 10 simple head constructions and thus used 10 times more of your time on effective training.
As for good beginner practice for still objects and landscapes alike, one essential link: drawabox.com. Go there, try it out, do it, even if it feels strange at first, it absolutely makes sense, and is explained much better, than this site or me or anyone else could explain to you in a shorter time.