18. (12 points) To investigate the mathematics exam scores of students citywide, 10 students each from Class A and Class B of a certain middle school were randomly selected, and the following scores (in points) were obtained.
$$
\begin{aligned}
\text { Class A: } & 132,108,112,121,113,121,118, \\
& 128,118,129; \\
\text { Class B: } & 133,107,120,113,122,114,128, \\
& 118,129,127.
\end{aligned}
$$
(1) Draw the stem-and-leaf plot for the mathematics scores of students in Class A and Class B, and determine which class has a higher average level based on the stem-and-leaf plot;
(2) If a mathematics score is 120 or above, it is considered "excellent." Find the probability that at most one of the three students randomly selected from these 20 students is excellent;
(3) Using the sample data of these 20 students to estimate the overall performance of the entire school, if three students are randomly selected from the school (with a very large number of students), and let $\xi$ represent the number of excellent students selected, find the distribution of $\xi$ and its expected value.
18. (1) Stem-and-leaf plot omitted, Class B has a higher average level.
(2) The number of excellent students in Class A is 5, and the number of excellent students in Class B is 6.

Let the event that none of the three randomly selected students are excellent be $A_{1}$; the event that one of the three randomly selected students is excellent be $A_{2}$; and the event that at most one of the three randomly selected students is excellent be $A$. Then
$$
P(A)=P\left(A_{1}\right)+P\left(A_{2}\right)=\frac{\mathrm{C}_{9}^{3}}{\mathrm{C}_{20}^{3}}+\frac{\mathrm{C}_{11}^{1} \mathrm{C}_{9}^{2}}{\mathrm{C}_{20}^{3}}=\frac{8}{19} \text {. }
$$
(3) Note that $\xi=0,1,2,3$, and
$$
\begin{array}{l}
P(\xi=0)=\frac{7}{95}, P(\xi=1)=\frac{33}{95}, \\
P(\xi=2)=\frac{33}{76}, P(\xi=3)=\frac{11}{76} .
\end{array}
$$

Therefore, $E \xi=\frac{33}{20}$.