Assuming the boulder Superboy is carrying is roughly a sphere with a diameter of 1.20 meters (3.94 feet) then the radius of the boulder is 0.60 m, or 60 cm.

The volume of the boulder would be:

4/3 π r³ = 4/3 π (60cm)³ = 9.05 x 10⁵cm³

The density of uraninite is 10.88 g/cm³, so the mass of the boulder is:

9.05 x 10⁵ cm³ x 10.88 g/cm³ = 9.85 x 10⁶ g

Uraninite gives off 1.98 millirems/hour/gram, so the boulder emits

9.85 x 10⁶ g x 1.98 mrem/hr/g = 1.95 x 10⁷mrem/hour

Now some fudge factors come into play:

The radiation exposure given above is the exposure at the surface of the boulder. I’m going to say that Ted and the teacher are twice as far back as Superboy was, so their exposure is one-fourth as much (thanks to the inverse square law). I’m also going to assume that the class is standing next to the boulder for five minutes (after all, who wouldn’t want to see the boulder Superboy flew in, especially since it allowed Ted to show up the spoiled rich kid).

Thus, Ted and the teacher received:

1.95 x 10⁷ mrem/hour divided by 12 (length of exposure) divided by four (distance from boulder), or

4.06 x 10⁵ mrem (406000 mrem = 406 rem = 4060 mSv = 4.06 Sv)

A CT has a 10mSv exposure, and a chest x-ray has a 0.1mSv exposure, so Ted’s exposure is equal to about 400 CT scans or 40,000 chest x-rays all at once.

An exposure of this magnitude is considered “Acute Radiation Poisoning,” with a fatality rate of 60%.

Now, I’m also assuming that Superboy found a vein of pitchblende particularly high in uranium ore content -- because he’s Superboy and he doesn’t do things by half. On the other hand, the average pitchblende is about 80% uranium ore, which would decrease Ted’s exposure by 20%, or make it 325 rem. This is a little better as it’s just “Severe Radiation Poisoning,” with a 50% fatality rate.