Chrys' Answer covers most of the ground I was going to, but one suggestion might be gamification.
From what you've said, some children see no value in going beyond basic maths, and while you might be able to convince them that those jobs they plan for require maths (good luck being a householder who can't balance a monthly budget), that's not always valid, and they can easily see counter-examples. It's even arguable that they're right and that the time spent learning maths is less valuable than focusing on what they're actually interested in (although I believe maths is interesting and fulfilling and that at least basic add/sub/mult/div is essential for day-to-day living, I can't dispute that most people can and do live happily in an algebra-free environment, let alone a calculus-free one).
So what you need to do is add "created" value to replace the "inherent" value that's lacking for them, and creating a game around it is one of the best ways to do that. (Hell, that's what a game does)
As an example, create a set of age-appropriate levels, with a set of SMART goals for those levels.
For example, everyone starts at a base level, and gains "experience" by answering questions in class, by getting above a certain grade in homework, and by being helpful in other ways such as staying behind to help clear (which gives kids who aren't as proficient a way to mitigate their struggling). The goals get more challenging as you progress (for example, XP would only be given for a 90%+ on a "Craftsman's" homework, whereas a "Novice" would get XP for any passing grade, they'd just get more for a 90%+.
As they rise in level, they get status and possibly first crack at whatever treats go on at the school (although this can backfire, see below), but you also have responsibilities to help out with other level students (Being seen as responsible and valued is a treat in itself for many children). This can be layered over a normal course pretty cleanly, as all you need to do is keep track of the XP.
Other benefits include being able to clearly see which children are struggling, being able to tailor for specific students in specific cases, and most importantly of all, a constant feeling of progress (normal grading being a bit prone to "you're only as good as your last grade").
Note that this is about competing against the tasks you need to accomplish, not against other students. Let them share their current XP/level if they want to, but don't have a "league".
How other students do should not affect your XP gain for several reasons, but the most important is that that kind of competition only "motivates the motivated". If you are regularly getting 10% lower grades than the more proficient students (who, for example, may have more supportive parents, private tuition, or simply a less stressful journey home, or who may actually be naturally more comfortable with maths), your reaction is more likely to be "fine he can have it", especially as they progress further. By making it about improving on your previous achievements, they've always got an achievable task, naturally tailored for them. As a bonus, this means that this technique can also be used even when there's only one student.
Obviously this alone takes a bit of work to manage, but if you're interested in deeper solutions, I can heartily recommend "Reality is Broken" by Jane McGonigal as a primer on this stuff (as well as a good read), and this video by the Extra Credits team. The video covers a lot of the stuff I did above, as well as mentioning a test case, and gives some great ideas for using the class dynamics to support and enthuse weaker students.