Partitions to different parts not exceeding $n$

In fact, you cannot prove unimodality of the coefficients of $P_n(x)=(1+x)\cdots (1+x^n)$ using the result about $B_m/G$ mentioned by Qiaochu. The $P_n(x)$ result is implicit in work of Dynkin on the principal sl(2) subalgebra of a complex semisimple Lie algebra. Hughes was the first to realize that a special case of Dynkin's result implied the unimodality of $P_n(x)$. Proctor removed all the Lie algebra theory from the proof, yielding a proof involving only elementary linear algebra. See http://math.mit.edu/~rstan/pubs/pubfiles/72.pdf, esp. equation (23). See also http://math.mit.edu/~rstan/algcomb/algcomb.pdf, beginning on page 90. It is a long-standing open problem to find a combinatorial proof of this unimodality result. I would be extremely impressed if a high school student could prove unimodality by any method.

This was originally an answer, but didn't work out; consider it a suggestion. The following is taken from a set of notes on combinatorics by Stanley I can no longer find online.

Let $B_m$ denote the poset of subsets of $\{ 1, 2, ... m \}$. Given a group $G$ acting on $\{ 1, 2, ... m \}$, let $B_m/G$ denote the quotient poset whose elements are orbits of the action of $G$ and where $x \le y$ if some element in the orbit $x$ is contained in some element of the orbit $y$. It is not hard to see that like $B_m$, the quotient $B_m/G$ is graded and rank-symmetric, with rank given by the size of the corresponding subset.

Theorem: $B_m/G$ is unimodal (the size of the ranks increase, then decrease).

The proof is somewhat involved; one defines certain linear operators between the free vector spaces on each rank. According to Stanley there is no known proof without linear algebra.

It seems plausible that the poset of distinct partitions into parts of size at most $n$ might be of the form $B_m/G$ for some $G$, but I have not been able to find such a construction.