RE: entry level board for kids
The Start S (Small) will work nicely for the smaller Kids (say ages 7-10 and not over about 100 lbs (45 Kg.) but for your larger kids the Start M would certainly be better.
What fins are supplied with your Starts? The Drake "Shallow 410 FRN"?
This is a 41 cm wide blade "cleaver fin" designed to give good performance in a fin that works in shallow water and for those who are not so tall when learning to beach start.
My guess would be that you have some sort of technique issue here.
I've used the Start and Rio M's for years with 2.5m2, 3.3m2, and 4.2 m2 rigs with no problems with the students tacking or jibing.
I'd suggest that the "critical issue" here is that you are not teaching them to place the foot of the sail all the way down on the back of the board. Also, it's good to teach them to "drive" the board up through the wind until the foot of the sail is all the way across the back of the board and pulled across until it rests near the outboard footstrap on what was the upwind side of the board.
Also, instead of having them step forward (holding the rig by the mast or the uphaul i.e. a rope tack) have them step over the mast keeping their feet and weight behind the mast foot and over the board's centerline.
In the "A Taste of Windsurfing" program we use this method very effectively and call it a "faster way to tack".
It works really well on wide boards like the Start, Rio, Cruiser, N Trance,
and Easy Rider, as it drives the board up through the eye of the wind and a little beyond, so all the student has to do with "sweep" the rig back out to perpendicular ( the T position) on the downwind side of the board and they are ready to step back, balance the rig across in front of their shoulder and rotate the upper body to add power and sail away.
The problem with small sails and using the more traditional "longboard" tacking method is that when the student steps forward, they bring the rig forward as well and the board stops turning as there is no "turning force" with the rig standing up because the center of effort/power (CE) of the sail is now standing upright over the mast foot, instead (as in the method mentioned above) keeping the CE of the rig well back where it has the power to drive the nose of the board up into and around beyond the eye of the wind (straight upwind).
Try the above technique yourself, with the 3-3.5 m2 rig and I think you will see how well it works.
Also, where are you placing the mast foot in the slot on your Starts?
I've found that it's very critical to put the mast foot all the way back as far as it will go, and only change it more forward for young sailors who seem to turn upwind all the time. It's easier to teach them to carry the rig a little more upright and forward to keep the board from turning upwind, but every once in a while you get a student that just turns upwind and stops all the time and for those students you may need to move the mast foot forward to perhaps the middle of the slot.
Hope this helps,