No transportation? A monthly pass for the local mass-transit system is usually pretty cheap, and with improvements in distance education, going to school every day will probably not be necessary in the future.
As for special needs kids?
I attend my local community college part time to keep in the habit of studying and to keep my technical skills up to date. Although most of the students are of sound mind (if not lazy), some are truly mentally handicapped, or otherwise severely impaired. While the naive inner liberal voice in me is rooting for their success, the bleak reality is that the vast majority of these students would fail miserably if held to the same standards as typical students. Even if they do graduate, they will never utilize their skills in any meaningful way because nobody in their right mind would hire them in a position of any type of responsibility.
So you're right, of course. A voucher is not going to be enough to pay for the education of someone with special needs. However, it will pay most or all of the expenses for most children, and by not subsidizing the education of special needs students to the tune of 10's to 100's of thousands of dollars that ultimately provides no real benefit for society, that money could go toward more productive uses: better facilities and equipment, better instructors, programs for adults that need retraining due to structural shifts in the economy, or simply back into the taxpayers pockets to be spent elsewhere in the economy.
And the special needs kid? They get a voucher like everyone else, and their parents can pay for additional expenses on their own dime if they want.
Yeah, I said it. Flame away.