Reduction from partition to multiprocessor scheduling
(This is the answer that the OP wanted to have checked.)

Let $S=\{a_1,...,a_n\}$ be an arbitrary instance of Partition. In our reduction we set $J=S$. if $\sum\limits_{a \in S} a$ is even, we set $m=2$. Note: If the sum of elements in $S$ is odd, there cannot be a Partitioning. Further, we set the length $l_i$ of job $j_i$ to the value of $a_i$.

So this is my reduction. But I am very unsure if this is just too easy? Note that I do not have to provide the correctness proof but just the reduction. Is this somehow a right reduction? I am very unsure if I have to distinguish between the two cases where the sum of elements of $S$ is odd or even. What if the sum is odd?

Can someone give me some hints on that?