It seems to me, the issues you need to address are two-fold; number one, the boyfriend and number two maybe she's been given too much freedom for the stage she's at in life.
The first issue I'd tackle is the boyfriend. You don't say whether you have a relationship with her boyfriend's family, or if he is still at home and in high school. At 18, he could conceivably be in his first year of college or just on his own, and already living the life of an adult in an apartment of his own, and possibly not accoutable to his family if he's paying his own way. This is a big deal since she will be influenced by who he's accountable to (if anyone) and where he's at with studies, and friends etc.
If he's still in high school, whether you know his family on a first-name basis or not, I'd contact his parents. I'd find out if he has curfew, what it is, and what their expectations are of their son when he's with a date. If there expectations don't match yours, I'd press upon the parents that you'd like their help in seeing that your daughter is home at an appropriate time. I'd also get their buy-in to allow you to call their house to check on her if its getting late and you haven't heard from her. If necessary, you and your daughter should sit down with the boy and his parents to talk about expectations, and why you think school should come first. Put them on notice and say that if they can't agree to your terms (curfew, expectations etc.) you will not allow them to date until school is out for the summer, and maybe not at all if she doesn't keep contact with you while she's out on a date. This should not be allowed at all. The first time she pulls this, I'd suspend dating privileges. Maybe scare the stew out of the guy and ask if he intends on marrying your daughter, and supporting her. You and she will know at least if he's a jerk or serious by how he reacts.
If this fellow is a bonified adult on his own, your work is really going to be cut out for you. You've already allowed her access to someone who she could conceivably leave home and move in with. She could be coming in contact with men older than he is or people who could be a bad influence, adult situations including access to bars, alcohol or illegal substances, sex and more. The fact that she's not upfront with you, and even lying would make me think she's definitely getting involved in situations that could impact her life in a not so good way. Reality is, she's pushed the envelope so far already, when you put your foot down, you could lose her if she gets mad enough.
I'd prepare myself mentally for it, and I'd still just sit down and have an honest talk with her. Ask her what she invisions her future to be. Ask her if she thinks she can really expect to get into college on the grades she's making this year. Ask her if she's even picked one out, and when she plans on applying. (When I was in HS, we had to get apps in by the summer of Junior year if were serious about any good colleges).
More important, I'd get honest about serious relationships too young. We all know that relationships can take an emotional toll on a young woman too inexperienced to handle the complications that come with it. Heck, adults can't handle complications of relationships, let alone a young woman. Plain and simple if she's serious about college and a career, a serious boyfriend at this stage of the game will sidetrack her efforts.
I'd Ask her if she's ready for the possibly becoming a young mother. If she'd be happy if she wound up working crummy jobs if she doesn't get into college. Ask how she'll feel if she never finishes college and winds up supporting a dead-beat boyfriend who may dump her when he's board with her. Show her a newspaper or just turn on the news and remind her that jobs are scarce and the economy tighter.. only the best will get into college, will get scholarship, will get jobs. I'd cinch the deal with a taste of real life and tell her she can do as she pleases, no curfew, no calls at all, but in order to do this she'll have to pay rent (at the current cost of an apartment today), pay for her own food, pay for her own gas when she uses the car, and pay for her cell phone/service, and any other things you provide for her.
In short, I'd get tough on her. Whatever it takes. YOu know her and her temperament best. But I'd be sure to not let her intimidate you. It's do or die. She's almost an adult and out of the house. I'd try to make the last year(s) count, and it starts with accountability and tough love.
Last but not least, I'd say unless she needs the job, I'd make her quit. It is a distraction from what should be her main focus--academics. It's also one more way for her to access her boyfriend without your knowledge. Do you really know her schedule? Perhaps she's not really working as many hours as you think.
I'd also consider establishing a hard curfew, a confirmed location where she plans to be that you can call if an emergency arises. This is not unreasonable in the era that we now live. You can even say this is for her own safety.
I'd threaten to cut off phone service if she refuses to answer it. This should be an absolute no. I'd also cut off access to the car if she doesn't make herself accessible once she's out the door. The phone may be good for emergencies, but if its becoming an instrument that allows her access to people, and situations that aren't good for her, it needs to go.
Save her before its too late. I hope some of these ideas help. Good luck.