Well, like you, I can't prove beyond the shadow of a doubt that the narrator is female, but I think the following four factors contribute to that overall impression:
* Author's name and photo (combined with the fact that the audience is children, who may not have had "the literary I is not necessarily autobiographical" drilled into them). Then again, the speaker is very clearly not the poet in lots of children's poems, so kids know lots of precedents for non-autobiographical "I"s.
* L1: Reference to someone named "Suzie," whose name also happens to appear in many classic rhymes and songs for jump rope and hand-clap games. These activities have a closer association with girls than with boys. But one could argue that this is an ordinary "little Suzie" rather than the legendary "Miss Suzie" of sailboat, etc., fame.
* The same line contains the clichéd rhyme "six" and "sticks"...which became clichéd by appearing in lots of poems and songs used for jumping rope...although one could argue that these appear in lots of children's songs in general (e.g., "This Old Man," "One, Two, Buckle My Shoe" [Edited to say--Oh, wait, I distinctly remember jumping rope to that one.]).
* L2: Sadly, "all the food I ate that day" is a concept more likely to be on the radar screen of girls than boys. From the infallible Internet: "By age 6, girls especially start to express concerns about their own weight or shape. 40-60% of elementary school girls (ages 6-12) are concerned about their weight or about becoming too fat. (Smolak, 2011)."
[Link to source] Again, one could argue that boys are infamous for eating prodigious quantities of food, and the reference could simply be to a large but indeterminate amount.
On an unrelated note, don't most kids outgrow their obsessive interest in dinosaurs long before they start encountering algebraic expressions with dual variables (x and b)? (I didn't, but was repeatedly told how abnormal this was. [Edited to say: I also loved math, and was repeatedly told how abnormal that was, too.]) I think the poem would resonate better with actual algebra students if it resembled the types of things encountered in "word problems" a little more closely--trains leaving stations, bathtubs filling at different rates--instead of having random references to primary-grade silliness like dinosaurs and little Suzie playing with sticks, which I never encountered in an algebra class.
[I think this is where I'd stopped when Roger mentioned my "last sentence," in his post below.]
The poem might end up falling between two chairs--young kids won't understand the algebra bits, and older kids will find the dinosaur and little Suzie bits too condescendingly babyish. But the silly humor and the anti-math sentiment may make up for that, even if the kids haven't encountered algebraic expressions before and thus don't really get the joke. I guess we'll see.