I have had this issue presented to me before. I would not allow the student who told me back into the classroom because they tend to become a liability. Instead, I redirect students to another teacher's room and have a fellow teacher notify the administration. There will be students in your room, and the student reported to have the weapon will be kept nearby until help arrives. The administration and SRO usually arrive very quickly before the student knows you know.

The most important thing to remember is to handle the situation before the student has time to react. Notify the appropriate authorities and remove the student from the class as soon as possible, don't allow them near their belongings.

I would call a neighboring teacher to watch over my class/direct my students to their room and take the student directly to the office. I want to get this student to the resource officer as quickly as possible, without giving the student too much time to react.

Ask the student who is reporting the weapon some follow-up questions. Did they actually see a weapon, or did they hear about it? What kind of weapon is it? Discreetly monitor the student who allegedly has the weapon while you're doing this. Once you have the information, call the SRO to come to your classroom and standby outside. It would be a good idea to have a code word/phrase that signifies what you're reporting without alerting the students. After the SRO arrives, you should approach the student who is supposed to have a weapon and ask him/her to step outside for a brief discussion. Make sure to keep the student's backpack separated from them when they get up to leave.

In a situation like this I would keep calm and go about this situation gently. If a student comes to tell me I would tell the student to keep calm and don't say anything to others to startle and scare the class because that could lead to an uproar in the class and the student who has the weapon might react in bad way because he knows that I know (the teacher) and the others(students) know he/she has a weapon. I would step out the class and make a quick call to the office so that the he/she with the weapon doesn't hear the call I would make to the office and explain the situation. Im sure at that point the office will send in the school deputy to handle the situation.

Go to each student individually and whisper to them that they need to get up quietly and go to the classroom across the hall.

i would get another teacher to take my students to safety while i take the student with the weapon to the office.

Get all the students to leave their belongings in the classroom and exit it to the hallway. Administration will then give you further instruction as to how to handle the situation.

I would first ask this student myself before referring them to the office, whether I have to take them myself or not. I don't think this is something that teachers so much have to take care of as it is making sure it doesn't get any further than just them bringing it.

remove the student immediately and once it is controlled, reassure the students.

I do not know if it is appropriate to do, but would it be the safest option to grab the backpack without the student noticing to secure the weapon while you wait for administration to arrive?