Please re-read Beth's good response below. I would second all that she says. Tell the driver directly and require that your daughter be seated directly behind the driver. Then report it in person (I would add, also in writing; have a brief and direct letter in your hand at the meeting) to the principal -- today. Do not let the school fob you off with "You need an appointment and she/he can see you next week." But also don't go charging in all angry or all emotional -- you will be taken less seriously.
In some school systems, spitting on another child once might get you discipline but doing it repeatedly would get you suspended. Schools know parents find spitting pretty invasive and disgusting and tend to take it seriously if it goes on repeatedly. It's treated almost like biting in some schools.
Do also give your daughter some tools of her own, such as teaching her to turn away or to make eye contact and say "NO, you cannot make a gun shape at me" or "NO, you cannot spit on me and you must STOP." She needs to learn to say it very loudly and firmly, which will be hard for her. Young kids are taught by us adults to be polite, use good manners, not be loud -- and that can have the unintended effect of making them think they will get in trouble if they raise their voices to tell another kid to stop a behavior. Role-play with her how to speak up, and also role-play how she should then turn to the nearest adult and say as firmly, "Those boys spit on me and they have to stop it NOW." Prepare her for kids to say "You're a tattle-tale!" and to respond that it is not tattling to keep yourself safe and she doesn't feel safe right now.
Still, she is five, they are nine, and among younger kids those four years are a huge age difference. These kids may indeed get worse when she does stand up for herself, though yes, she does need to stand up for herself; however, I agree with Beth's post that your daughter at five needs some additional backup -- namely, you (and dad too if possible) seeing the principal today. I hope you have these kids' names, by the way, as that makes it faster for the school to deal with this.
Do not leave any meeting with anyone at school until you have said, "Now, exactly what step will you take about this, today?" Then sit there and look expectantly at them, clearly waiting for an answer. They will not commit, in your hearing, to any specific thing like "We'll suspend them" etc. -- they simply cannot discuss, with you, discipline for a child who is not your own. But ask the question and wait for some kind of reply, and tell them that you will be calling the next day to ask what is happening.