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I think from the comment that the suggestion was that halves use tfrac and not that all fractions use tfrac. The latter is clearly unmanageable as the fonts simply become too small when fractions exist within fractions. Keep in mind that this is about using \LaTeX\, to teach which means showing steps in working out problems in the clearest way possible.
I could almost live with the effects of tfrac at the beginning of the problem, though I don't like having items at the same level in fractions and super/subscripts of different sizes for the following reasons. Making the half small somehow implies it's not really an important part of the equation when in fact it is a quite important part not only numerically, but in understanding that each of these terms came from integrating a linear function leading to a quadratic with a factor of one half in front. Furthermore, factors of a half cannot be found by dimensional analysis so they need to be very visible and therefore very memorable.
\begin{align}
& mgh = \frac{1}{2}mv^2+\frac{1}{2}I\omega^2 \qquad\mbox{ using frac} \\
& mgh = \tfrac{1}{2}mv^2+\tfrac{1}{2}I\omega^2 \qquad\mbox{ using tfrac}
\end{align}
But a line or two later in working out the problem that becomes this and now the tfrac doesn't make much sense.
\begin{align}
& gh = \frac{1}{2}v^2 + \frac{1}{2}\frac{2}{5}v^2 \qquad\mbox{ using frac} \\
& gh = \tfrac{1}{2}v^2 + \tfrac{1}{2}\frac{2}{5}v^2 \qquad\mbox{ using tfrac}
\end{align}
Other situations are even worse. Why for instance would I imply that the $ 2 $ is somehow different than other quantities in the denominator in the following expression by making it a different size?
\begin{align}
& \frac{1}{2}\frac{\mu_s}{1+\mus_s\cos(\theta)} = \frac{\mu_s}{2\( 1+\mus_s\cos(\theta) \)} \qquad\mbox{ using frac} \\
& \tfrac{1}{2}\frac{\mu_s}{1+\mus_s\cos(\theta)} = \frac{\mu_s}{2\( 1+\mus_s\cos(\theta) \)}\qquad\mbox{ using tfrac}
\end{align}
In the last example, there is no good mathematical reason for the two to change size across the equals sign.
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