I think your ability to be able to become grounded / stop being dissociated depend on why you are dissociating to begin with - ie, I very much believe it depends on the type of trauma you experienced.
For me, I cannot tolerate ANY form of 'grounding' in any of the main ways other people seem to be able to. This is ESPECIALLY true for me when I am with or around other people. In therapy is the worst place and thankfully neither my pdoc or my T will ever even try to "help" me ground. Why? Because I am dissociating because I feel absolutely UNSAFE in the present. I am dissociating BECAUSE I'm with other people - 'grounding' me or 'helping' be come back to the present makes the dissociation worse. Something about that situation left me so unsafe I second in the only way I knew how - if I felt unsafe being there (and therefore dissociated) it makes no sense at all, to 'come back' until I am ready.
I don't think it will always be like that. My pdoc said I'm simply too traumatised yet - but as I work the my traumas, I do expect I will become more tolerable in 'being with' others while in a highly distressed state. My key trauma was child abuse at the hands of my caregivers - my T tells me it's perfectly understandable I'd struggle the most with emotional intimacy and feeling safe, when in the midst of dealing with this.
I don't know what yr trauma was - but if it related to being abused or made unsafe in the presence of people you trusted deeply and then they hurt you in unexpected ways, it would make sense you don't necessarily feel safe enough to 'ground' when around other people.
On the other hand - at the very roots, dissociation is a symptom of severe anxiety. Do you find you are triggered and then dissociate? Is it being at school that is triggering the increased anxiety? Or something particular about driving?
To a certain extent, people who don't have PTSD dissociate while walking or driving. They daydream / space out. Mis it I referring with your life badly?
Some techniques I've heard that work for other people:
Snapping a rubber around your wrist. Ie wear it around your wrist and when you need to ground yourself, pull it so it pings back. A similar one is using a block of ice to hold in yr hand.
It sounds like you've done the 'naming things in your environment' - my old T used to say it's important to focus in the different senses - ie, name 5 things you SEE, 5 this you HEAR, 5 things you SMELL.
Saying a phrase over an over "it is all ok, I'm not going mad, it's just anxiety" or "I am safe".
When I'm alone I try these things (when having a panic attack). I lie down in the ground on my stomach. The firmness of the ground leaves me feeling safe.
I also hide under a blanket.
I go to sleep. My body takes over and I am hit with sudden exhaustion and HAVE to SLEEP. Yet I'm sleep with my eyes closed but mentally alert. I can't move or open my eyes, but I feel very calm.
Other coping strategies I use for anxiety: watching tv; being online; I especially find listening to online program's calming (Dr Phil!!) while I'm doing some other activity (paperwork or dishes)
I also find colouring in / doodling / scribbling helps.
Having a shower and washing my hair.
Changing the sheets on my bed - heavenly to climb into bed after a hard day mentally, with the smell and feeling of clean fresh sheets!
Hope some of those ideas might help. Remember - dissociation is not you 'going mad' - it's 'just' a symptom of anxiety :) (Not trying to invalidate your distress - I just know for me it helps to remind myself that when when I'm dissociating - because I start thinking I'm going mad and then the fear and anxiety REALLY takes off - and round in cycles I go).