Any number inside the modulus sign becomes positive. This means
and so we have,

Solving these gives us


However if we check the second solution in the original equation we obtain
. This is false and so
can't be a solution.
Therefore the only solution is
.
(Note: I'm not sure why the second solution didn't work but when there's a modulus sign involved it always pays to check your final answers to be sure. I'll have a think about it but in case you find out before I do, I'd be interested to know in the comments.)
Let's call the younger student's age A and the older student's age B. The teacher's age will be T.
B = 2A
T = 5B
T+5 = 5(A+5)
Simplify the last equation.
T+5 = 5A+25
T = 5A+20
Now we have two equations solved for T, so we can set them equal to each other.
5B = 5A + 20
We can plug 2A in for B
5(2A) = 5A + 20
10A = 5A + 20
5A = 20
A = 4
To find T, we plug 4 in for A in T = 5A + 20
T = 5(4) + 20
T = 40
The answer is 40 years old.