Remember that the sharp scissors probably belonged to the school - teachers have a need for scissors too. You do want to raise the question of how 'DP' got his hands on them though.

I'd suggest that starting at the bottom of the chain is a good idea. Teachers tend to be overworked - 30 kids is a handful whichever way you look at it, and going immediately up the ladder is not going to help get the teacher onside. It's possible (probable?) that the teacher remains completely unaware of the scissor situation, and it's not fair to let him/her be in that situation whilst you go the the headteacher.

That being said, the cloakroom situation is the headteacher's domain. So you do need to speak to them too.

I'd suggest having a quiet word with the teacher first; Bring the scissor situation to their attention, and inform them that you need to talk to the headteacher about the other situation anyway, and that you intend to discuss the scissor situation at the same time. The teacher may well get defensive - just be calm but insistent. If the teacher says that he/she was completely unaware of the incident then accept that at face value, but don't let him/her talk you out of speaking to the headteacher.

At the end of this, the teacher needs to know;
1) About the incident.
2) That you appear to be a reasonable and calm parent looking to resolve the situation, and not on a witch hunt.
3) That the incident is going to be be further discussed with the headteacher.

Then set up a meeting - yourself, the headteacher, the teacher and the ex-helper. Arrive with an agenda; 1) The cloakroom incident, 2) The scissor incident, and 3) A proposed action plan.

It may be that the headteacher is not willing to accept the ex-helper being privy to the whole meeting, (due to their 'ex'- status, and the students' rights to privacy). But you do need to get their version of the cloakroom incident accepted into the meeting early on - it validates your concerns, and is likely to result in the (unseen) scissor incident being taken more seriously.

The proposed action plan should be realistic, well defined, and time-limited.

I'd suggest;
1) The teacher agrees to ensure that any potentially dangerous implements are stored safely out of reach. (Note the wording is important - you want the teacher to readily agree to do something *that they should already be doing* and to avoid placing any blame on the teacher for the past incident)
2) The teacher agrees to pay closer attention to interactions between 'DP' and other children (in particular your child), and to keep the headteacher updated weekly.
3) The head teacher agrees to take notice of those reports, and take action when necessary - a first step might be to provide extra classrom assistance to the teacher. If this kid really is dangerous (as opposed to just a bully), then to take immediate action to ensure the safety of other children in the classroom.
4) You agree to bring any other incidents that you hear about to the headteacher's attention in a timely manner.
5) The head teacher agrees to give you an update of any issues at half term.
6) The head teacher agrees a further conference to be held at the end of term, at which point either the issue has been dealt with, the 'watch' needs to be continued, or further action is necessary. (Ideally the first..)

The issue that I haven't addressed at all here is that of 'DP''s parents. They have a need to know that their kid is being scrutinised. Not only that, but they could be the solution to the problem. I'd suggest leaving that to the headteacher. (In fact, maybe make that another action point).

The point of the action plan being time-limited is that you just cannot put another family's kid under a life-time watch. It won't achieve anything but restrict your actions for escalating any issues. Having the plan with a defined end point defines when it's time to see whether the plan is working, and when to take further action if necessary.

At all costs, avoid losing your cool, or placing blame on the school or teachers. You need them to be proactive in resolving the issue, not thinking that you're Mr Unreasonable, nor covering their butts.

I'll leave you with a few thoughts;
1) Everyone is bullied in some way at some point in their lives. Learning how to deal with bullies is an important part of growing up. (That's not to say that dangerous or injurious behaviour should be condoned...)

2) Everyone has to learn what behaviour is acceptable and what is not at some point in their lives. Different people will obviously learn this at different times, and the triggers and methods of this learning will also be different. Don't assume that 'DP' will always be a bully.

3) It is far better for the issue to be resolved within the school if possible. Every escalation increases the chances that 'DP's life degenerates into a repetitive cycle of school exclusions and violence, something which I think that everyone would like to avoid.