Are these rules for my school necessary?

Most secondary schools have this rule, so your school definitely isn't the only one in this. A lot of it just goes under discipline, and 'preparing' you for the future. It obviously does seem pointless as a lot of these are stripped at sixth form and university. Uniforms do help identify you as a student if you are out of school grounds though and it is an identity thing too. For phones, that falls under the umbrella of a lot of things. Most schools started banning them because of distractions and people not paying attention especially when there are less serious people at secondary school. I'm not sure if your school completely bans them, ie you're not allowed to bring them into school at all or just prohibits the use of them but they are also a safety and consent concern. Since you're under 18, the school is responsible for what you do at school whether that's online or in-person including on your phones and that's why the cyber security thing is also a concern. They also can't fully control or invigilate what every student is doing on their phones and then there's also the concern of people taking photos without consent. Unfortunately schools nowadays have a bunch of regulations and rules that they have to adhere to even if they're ridiculously illogical.

When I was in school these were the rules as well, and I don't blame my secondary school for enforcing them. Blazers make you part of the school you're in and not wearing them like everyone else makes you come across as rebellious.
I don't think mobile phones are necessary, if students are worried they're in danger they can contact teachers and staff, or otherwise find friends, acquaintances or confidantes to hang around with to keep them safe. Phones don't help to solve problems when it comes to being in imminent danger, so you need to use what's in your environment and stay with people who you trust.

In my secondary school, it is compulsory to wear our blazer. We’re only ever allowed to take it off during lesson time (with teacher’s permission, they usually allow you to take it off) and in summer (if the headteacher says it’s too hot). The headteacher says it’s for formality but they are quite uncomfortable and many other schools have it as an optional part of their uniform. Another rule is, mobile phones are completely prohibited. It is said that this rule is to prevent cyber bullying around the school and to cross roads safely (I guess some people have their eyes glued to their phone whilst crossing the road). While this can be seen as fair and could be a safety issue, there’s still reasons why they should be allowed. What if our parents are worried for our journey to school and/or on our way home from school? What if we suspect we’re in danger on our way to school and/or on our way home? There can obviously be things like: not being allowed to use it in lesson and it must be on silent. Overall, I think these rules are quite unnecessary. Would you agree?

My school does not have blazers, so I cannot say much there, however not being able to even bring your phones to school seems like a big hazard. What if you need to call 999 while on the bus ride home, or you need to send your parents your location so they know that you're safe? Of course, the rules around phone usage in school make sense, you're meant to be focusing on studying, not phones, but not being able to use your phones while in the streets doesn't. And say there is someone lives really far away from school and they take multiple buses, and there is absolutely no one who takes the bus with them, if they do not have a phone to call their parents in an emergency, anything could happen. My school just tells us to make sure that they're off and either in our lockers or in our bags, if people use them they get a detention.

Bear in mind people used to go to school and home just fine before mobile phones.

That's completely true actually, I hadn't thought of that.
Still, not being able to use phones in the small chance of an emergency is a bit of an unnecessary rule.

In my secondary school, it is compulsory to wear our blazer. We’re only ever allowed to take it off during lesson time (with teacher’s permission, they usually allow you to take it off) and in summer (if the headteacher says it’s too hot). The headteacher says it’s for formality but they are quite uncomfortable and many other schools have it as an optional part of their uniform. Another rule is, mobile phones are completely prohibited. It is said that this rule is to prevent cyber bullying around the school and to cross roads safely (I guess some people have their eyes glued to their phone whilst crossing the road). While this can be seen as fair and could be a safety issue, there’s still reasons why they should be allowed. What if our parents are worried for our journey to school and/or on our way home from school? What if we suspect we’re in danger on our way to school and/or on our way home? There can obviously be things like: not being allowed to use it in lesson and it must be on silent. Overall, I think these rules are quite unnecessary. Would you agree?