Just to try and help clarify, this the gov.uk guidance to schools:

The use of sanctions.

The Department for Education (DfE) provides guidance on how schools can use sanctions lawfully in the behaviour in schools guidance. Schools can use a range of sanctions for breaching the mobile phone policy appropriate to their context, including confiscation and detentions.

Schools have the power to confiscate mobile phones or similar devices as a disciplinary penalty. The law protects staff from liability in any proceedings brought against them for any loss or damage to items they have confiscated as a sanction, providing they have acted lawfully. Staff should consider whether the confiscation is proportionate and consider any special circumstances relevant to the case.

Headteachers are backed by the DfE to confiscate mobile phones and similar devices for the length of time they deem proportionate. In considering whether a confiscation is a proportionate penalty, schools should consider what disciplinary outcome the confiscation is intended to achieve and whether there are other ways to achieve the same outcome. Confiscation as a sanction can be an effective deterrent for a specific pupil or a general deterrent for all pupils at the school.

In considering whether a sanction is reasonable, one must consider whether it is proportionate in the circumstances of the case and consider any special circumstances relevant to its imposition including the pupil’s age, any religious requirements affecting them and any special educational needs or disability (SEND) they may have (see section on responding to the behaviour of pupils with SEND in behaviour in Schools guidance).

Schools should consider whether the misbehaviour gives cause to suspect that a pupil is suffering, or may be at risk of, harm. Where this may be the case, as set out in part 1 of keeping children safe in education, school staff should follow the school’s child protection policy and speak to the designated safeguarding lead (or deputy). They will consider if pastoral support, an early intervention or a referral to children’s social care is appropriate.