More needs to be done to improve education for children who are not in mainstream schools, inspectors have warned.
Youngsters who attend "alternative" education settings full-time sometimes study a very narrow range of subjects, and only take very low-level English and maths qualifications, according to a new report by Ofsted.
The study, based on a survey by the schools watchdog of 165 schools and the 448 "alternative providers" they use for children who cannot be in the classroom, concludes that more needs to be done to improve the education and safety of these pupils.
Overall, more schools are refusing to use providers that they do not think are good enough, inspectors found, while alternative providers - which teach the most vulnerable children - are usually safe, with decent accommodation and resources for pupils. But the watchdog warned that some youngsters may be missing out, particularly in English and maths.
The survey showed that some pupils were still missing out on English and maths teaching at school on the days when they attended their alternative provision, although the picture now is better than in 2011.
The report said: "There were substantial gaps in some pupils' timetables in almost a tenth of the schools visited, either with insufficient provision for English and mathematics, or timetables that were too narrowly focused on a very few activities across each week.
"When pupils do miss key subjects, they can find it very difficult to catch up. As a result, they often underachieve and do not gain the qualifications they should.
"Pupils who attended alternative provision full-time sometimes studied a very narrow range of subjects, and the English and mathematics qualifications they were enabled to take were only at a very low level. Occasionally these pupils did not receive a full-time education."
The report also notes that there is no specific reference to alternative providers in the latest government guidance on safeguarding.
"This situation sometimes leaves schools uncertain about what is required and what would be considered to be good practice with regard to checks on alternative providers," inspectors said.
Alternative providers that offer part-time education, or full-time education to small numbers of pupils, do not have to be registered, the report notes, adding: "This remains a concern and means that pupils can spend all or the majority of their week at a placement that receives no external inspection or regulation.
"Schools are responsible for ensuring the quality of the placements to which they send their pupils. Most of the schools in this latest survey did assure the quality of the provision reasonably well, but there are risks involved in this situation, particularly as many of the pupils who attend alternative provision are the most vulnerable and disenfranchised in our education system."
The report sets out a series of recommendations, including calling for schools to carefully check the registration status of alternative providers, and to give these providers good quality information in writing about the school's expectations on child protection.
Sean Harford, Ofsted's national director for education, said: "Schools are voting with their feet when it comes to poor providers. More and more often we see schools working together to identify and commission better alternative provision.
"However, it is vitally important that schools recognise their responsibility for each and every pupil sent to an external provider. These are some of the most vulnerable children in the education system and the school is responsible for ensuring their personal and online safety while they are off site, as well as the quality of the education provided."
A Department for Education spokesman said: "Every setting should be delivering an excellent education while ensuring the safety and well-being of pupils, and we welcome Ofsted's recognition of the progress we have made with alternative provision.
"However, we recognise there is still work to be done to make alternative provision even more rigorous, and we will be coming forward with plans in due course."