This is a point that I've heard both sides of in the past few years. When I became Muslim (almost 5 yrs ago, alhumdulilah!), I was taught by another sister that it is okay to wipe over your hijab, much like we do wipe over our socks- IF you have made wudu before putting them/it on in the morning.
When I started teaching a couple years ago at an Islamic school, one of the teachers brought to my attention that the girls were wiping over their hijabs, and said how it's not allowed. So I just gently reminded them to take off their hijabs when making wudu for prayer at school. But then the mother of one of the girls, who also happens to be the current principal (vice principal at the time), told me that I shouldn't have done that, because in fact it's allowed to wipe over hijabs out of necessity to make wudu go faster. I told her that I was taught that when I became Muslim, but another teacher had brought to my attention that it was incorrect.
As recently as a few months ago, this issue came up again just after one dhuhr prayer at school, when we were telling the children they can't wipe over socks if they didn't have wudu before putting them on, or if they were dirty from wearing several days, etc, and I made a comparison to making wudu over a hijab and then taking it off. One of the other teachers told me that she's pretty sure we can't wipe over our hijabs, but it is allowed to women with long hair to wipe over a braid, for example, instead of undoing her hair each time and redoing it.
So I'm really confused on this issue. Since it was brought up again in the past few months, I've been following the "take off the hijab for all wudu" guideline; something I didn't always do in the past. And really, it's the best way to make wudu anyway. But for clarification for the students, does anyone have hadith on this that is solid?